Temporally, there is much for which to praise the film.  It uses many of the ideas others have explored for "what would you do if you had a time machine", and devises some I, at least, had not previously seen.  Unfortunately, it draws tropes from other stories and combines them in incompatible ways--most notably putting fixed time tropes in what is evidently a replacement theory universe.  Then it ties up the loose ends with concluding events that are both a major temporal disaster and a self-contradictory impossibility.  It is also inconsistent with its own rules, making it difficult at times to know what is happening.

It opens early in 2014, near the end of David Raskin's senior year of high school as he creates his application for a Massachusetts Institute of Technology (MIT) physics fellowship, and is accepted but underfunded.  His mother's response to this is to put the family home on the market, and David starts searching the house for anything his father might have left unfinished which he might be able to turn into a project worth funding.  Christina joins him in the attic, and they find a video camera on which their father apparently recorded events of David's seventh birthday--the last time David saw his father alive.  David watches the video, but then sees something unexpected:  an image of himself, at his present age wearing his current favorite shirt, reflected in the mirror at the party.  He shows it to Christina, and then to his two "engineering nerd" friends Adam Le and Quinn Goldberg, seeking some explanation for how he, at roughly his present age, could be in a video that has been hidden in his attic for a decade.

In attempting to solve this mystery, they notice two things about him.  One is that he is holding something unfamiliar in his hand which is identified as a key chain that does not belong to any of them.  The other is that he appears to be operating the switch that controls the basement light.  This leads them to the basement, and then as someone plays with the lightswitch upstairs they hear a noise which brings them to a hidden lockbox in the floor (solenoids secure the bolts when the light is powered, release them when it is extinguished).  Within this they find parts and diagrams for something called "Project Almanac" (from which the film gets its name), the description of which ends with the words "temporal relocation", which, as David explains for the benefit of anyone who did not get it, means time travel.

The foursome discuss whether they are going to build a time machine, and one of them, referencing the video, opines, "I think we already did build it."  That, though, is the beginning of our temporal difficulties.

There is a significant problem in the story already:  a major part of their present situation is the result of the fact that David saw himself in that video.  The obvious impact is that they have the motivation that comes from the belief that they have already succeeded.  That is reflected in the response to the question of whether they were going to build a time machine, "I think we already did build it," and again when Jessie "Jess" Pierce is added to the group and she challenges them that they already know that they can make the trip because they already have the proof that they will have done it (overlooking that they might all but David have died trying to make it work, as only he is seen in the video).

This is a variant of a predestination paradox, in which something in the past is caused by a future event which is in turn caused by the past event.  In this case, because David has video of his appearance in the past he makes the effort to get there and also makes leaps he might not otherwise have made had he not had this proof.  Yet it is more complicated than that.  Because he has Jessie's keys in his hand, she has a basis for pushing her involvement in the group when they are identified; that happened simply because he had access to her car and used it just before his abrupt departure on that final trip, but he had the car because of their relationship, which happened because she became involved in the time travel project.  Yet there is an even bigger predestination paradox here:  the light switch.

David found the lock box in the floor because he heard the solenoids when someone flipped the switch; he had them flipping the switch because in the video he saw his own hand reach for that switch.  Why, though, did he do that in the video?

The switch was either on or off.  If it was off, then logically he must have been turning it on in order to have light in the basement; if it was on, he was turning it off in order to unlock the lockbox.  However, we learn eventually that David's father was already in the basement at that moment.  If someone flipped the switch, the lights would change status--either from off to on or from on to off.  Yet when the switch is off, the lights are off, and while it is likely that there was another light in the basement on a separate circuit, when David's father came downstairs he would have turned on the light at the top even if he then turned on another light below (and when David was looking for the lockbox, why did he not ignite the other lamp?).  So David probably found the lightswitch in the on position, and probably would not have doused the lights before descending the stairs (there is no sign of a flashlight, and the arrangement of the cluttered basement ten years ago is likely to be a bit different than he remembers)--and the light is on when he finds his father down there.  That means that there is no reason for David to reach for the lightswitch unless he does it so that he will see himself do it.

This makes perfect sense in a fixed time story; this, though, makes itself very clear that the time travelers are changing history, and changing their own history.  For such a paradox to work in a replacement theory story we have to find an original cause.  Perhaps the original David, the one who did not know his actions were going to be seen by his younger self on the video, reached for the switch to ignite the light and then saw that it was lit.  The fact that that action was important was incidental.

Besides, there is a much bigger problem with that segment, which we will eventually address, that makes any effort to resolve that paradox moot.  We have a lot of other ground to cover first, though.

There was something reminiscent of Primer in the credible approach to the experiments, although perhaps the idea of putting the stopwatch on a toy car was a tip of the hat to Back to the Future.  We are given a date, that by February 18th, 2014, David Raskin believes he is building a time machine.

The power supply problem is an interesting quirk.  It has a certain appeal in that they address it with batteries, and ultimately with the metal hydride battery in Jessie's car (which is how she becomes involved), because the electric car industry has been a major driver of improved battery technology over the past decade (in something of the same way that the space program of the 1960s was a major driver of miniaturization in electronics).  It gives us a plausible reason why David's father did not have a working time machine yet David is able to complete and operate the device without more than the instructions given.  It also connects Jess to the group, because it is her car they use for the power supply:  a battery that can recharge, be regenerated.  Of course, it does not really make much technological sense:  a transformer and bridge rectifier array would provide many times as much power from a standard wall outlet as any battery in any electric car.  Besides, any advantage they might have gotten from a better battery they will have lost in the poor transmission of long chains of jumper cables:  even the good heavy ones do not connect well, which is part of why it is so difficult to jumpstart a car from a perfectly good battery.

The next step is also a misstep, as to go beyond the car they decide they need hydrogen, which they have to steal from the school lab supplies because you need a license to buy it.  Someone says something about needing it for fusion.  On the last point, hydrogen is used for fusion, but to date no one has managed a fusion generator system that produces more power than it consumes.  Meanwhile, David and his friends could easily create their own hydrogen--people have been making it since at least the nineteenth century by capturing the effervescence from rust, it is the effervescence of most acid reactions including the gas produced by a standard lead-acid automotive battery, and can be made by electrolysis.  Apparently the filmmakers were hoping you would not know how ubiquitous hydrogen really is.  But then, just as they needed a way to bring Jess into the group by having them use the battery in her car, they needed to make the time machine dependent on some depletable but obtainable resource, something that in the critical scene David would have to go get in order to make his final trip to the past.  It also gives the film something that allows David to improve the abilities of the machine gradually over time without requiring that he ever understand how the machine itself actually works:  he finds ways to improve the power supply for it.

The first successful test is not well explained.  What seems to have happened is a toy car was placed within the field with a stopwatch.  The machine was then activated, causing a blackout (why, if they are operating on their own power supply?) but sending the target object to some unstated time presumably in the past.  A moment later they find the object, and the clock on it tells them that the car has been in the basement longer than the other stopwatch they kept with them.  From this they conclude that they sent it to the past, although they are going to need to do a bit of calibration, since they were aiming for sixty seconds and got two hours.

They then make one of those leaps we discussed, ignoring their better judgement regarding experiments with inanimate objects and simple life forms and, at the insistence of the least scientifically oriented member of the group, going directly to the first human test.  This probably saved screen time, but it creates potential disaster:  without that video image that they cannot have the first time, they will almost certainly use caution and delay the human trip.  This gives us a much more complicated timeline, but not one impossible to resolve, as they will simply cut out tests with an earlier human trip, creating a satisfactory stable history.

The first experiment demonstrated that they would be able to send an inanimate object back in time a few hours.  Then, relying on the video, they leap to the first human experiment, as the five of them (David, Jessie, Christina, Adam, and Quinn) leap back one day and return.  What is immediately confusing is, how did they do that?  The time machine, such as it is, is a rather large array of equipment on a table in the Raskin basement, and the power supply for it goes from being a huge collection of batteries to being the battery in a car and then a very heavy high-tech battery David bought somewhere.  It is simple enough to understand how they would be sent from the departure point by a powerful time machine--but how do they get back home?

The answer appears to be that David miniaturizes something.  The problem is, what has he miniaturized?  If he takes the time machine with him, how is it powered?  If he has solved the power problem such that it can carry a battery sufficient for the purpose, why does he need the larger battery in the basement?  There seem only three possibilities.

The first is that the device that is carried into the past is only part of the time machine, that when it is activated it works in concert with the part in the future to cause the time travelers to be drawn back to their point of origin.  This, though, is not distinguishable from a recall device, and we have discussed the problems with such devices in examining Timeline (twice) and Timecop:  when you change history by activating the device in the past, you must then live through all the history up to the moment in the future when the signal is received and the future device is activated, and only then will history be altered by removing you from it.  Of course, the time travelers are deprived of all recollection that this happened because their experience is erased, but people who are unexpectedly stuck in the past have a high probability of doing something that will prevent them from making the trip to the past, with severe consequences.

The second possibility is that a return trip to your own time does not require as much power as a trip to a point in the past.  There might be a logic to this, as we are already traveling forward in time so it would be a simple matter of accelerating the rate of travel rather than reversing it.  Oddly, though, never once do the time travelers consider trying to reach tomorrow.  They travel to the future repeatedly, of course, but it is always the future from which they came.  Perhaps it simply did not interest them.  Perhaps the film is relying on the foolish trope that you cannot travel to the future because it has not yet been written (foolish, because in that case when you travel to the past you must erase the future and rewrite it, and you are stuck in a past for which the future has not yet been written).  Yet it is possible that the backpack carries enough power to carry five people (and a dog) forward in time but not enough to travel back.

The third possibility is a bit tricky.  Let us suppose that when the time machine is activated it opens a corridor stretching from the activation point back to the destination.  That corridor now exists.  The question of for how long it exists is meaningless:  it exists in every moment of time between the departure and the arrival points.  At the departure end, it need remain open only a few seconds.  The travelers enter the corridor at the future end and exit at the past end, and then as they live in the past they are traveling, temporally, parallel to the corridor.  All the backpack machine needs to do is enable them to reenter the corridor and move to the future end.  They will of course in some sense encounter themselves--just as the corridor exists in every instant of time, so too the time travelers within it must exist in every instant of time as they travel to the past, and again distinctly as they return to the future.  The problem with this solution, though, is that every time they make a trip to the past they create another corridor, and if two corridors exist in the same moment of time (that is, if they create overlapping corridors and are present during a time when they overlap) there is no clear way to control which corridor they enter, and thus no way to know at what future point they will emerge.

That might be the case, though, as there are sometimes problems with the point to which our time travelers return, as we will see.

The film gets credit for its exceptional use of butterfly effects:  whenever they return to the future there are differences that are not always obviously consequent of their actions.  In the first trip, they are chased by a dog who rushes into the field with them when they are returning, and as they return they find the street plastered with missing dog posters.  That one was obvious, and told them that they were changing the past.  They make numerous intentional changes to the past, but more significant are the incidental changes.  After their trip to Lollapalooza Christina is one of the popular girls at school and Mrs. Raskin has a job; after David makes his first solo trip his mother never had the job but has two interviews.  David's solo trips have serious unexplained consequences.

The best of these from the perspective of our understanding is given to us as a chain of events.  When they return from Lollapalooza (which according to Wikipedia was held in Chicago every August since 2007, on the 2nd through 4th in 2013), they discover that a plane crashed--an event which had not happened in their remembered history.  It seems, though, that the pilot was the father of someone in their school, Sarah Nathan, whose brother was on the basketball team.  In their memory the team went to the playoffs and the pilot took the night off work for the game; in the altered history, the captain of the basketball team, Justin Kelly, broke his leg when he was hit by a car at a party, so the team did not make the playoffs, so the father was flying the plane instead of watching the game.  The chain is obvious once it is identified--with the only gap the question of how it happened that the kids traveling from Atlanta, Georgia, in February/March of 2014 to Chicago, Illinois, in August 2013, and back resulted in the captain of the basketball team breaking his leg.  That, though, is exactly right for a butterfly effect:  we know that these people are connected, and a slight change in their interactions at some point will ripple through the world and change when they are where.  The boy was hit by a car, apparently, which means he was in the wrong place at the right time--a place he had not been in the world in which David never attended Lollapalooza.

The explanation of this one, as far as it goes, does much to illumine the others:  if you travel to the past, you are likely to change the world in ways you could not have anticipated.  You were seen; people changed their path to walk around you; you replaced oxygen with carbon dioxide, released moisture and odors and gases into the air, set up ripples of sonic and pressure energy and in turn absorbed and redirected some that were there.  You might have done everything you could to avoid having any impact on the past, but you cannot visit the past and not alter it.  We never understand specifically why the changes that occur are consequent of the trips that are made, but that is the essential point, and it is well done.

When Ray is giving the rules for time travelers in Frequently Asked Questions About Time Travel, one of them is "Don't touch yourself," clarified as your other self, your temporal duplicate.  This is indeed a trope of time travel, which we have considered previously in connection with Mr. Peabody & Sherman, Timecop, and Back to the Future II, and which was even a point in an episode of Doctor Who--but as we have seen is not the kind of problem usually imagined despite giving us the kinds of potential problems we saw repeatedly in Predestination.  Project Almanac also uses the idea, doing something different with it--but possibly not something credible.

On their first human trip to the past they travel to the previous night.  Wanting to ascertain whether they are actually when they think they are, Adam has an idea.  He lives nearby and should be asleep, and his mother will be asleep, so they quietly enter his home, his bedroom, and see him lying on his bed.  Adam then draws on the back of the neck of his younger self.

The film blatantly borrows from Looper, but in a perfectly logical way:  Adam asks whether his companions had seen the movie, and then as he draws a dark line on the back of the neck of his younger self, we see the line appearing on his own neck.  The idea is obvious:  if you change who you were then, you change who you are now.  Unfortunately, this is one of the genuinely weak points of Looper, on which it proved inconsistent and which ultimately destroyed the ending of the story.  Just because you saw it in a movie does not mean it works.  Besides, if he was assuming that what happens in other time travel movies is correct, he should have considered the example of Timecop and been more careful about what he was doing.

It is indeed an odd conception.  Adam enters the bedroom and sees his younger self, and nothing happens.  Then he touches his younger self, holding his head still while drawing on his neck, and does not experience anything negative.  It is not until the younger self sees him that something bad happens.  It is described as a feedback loop which causes both versions of Adam to flicker.  The team drags him out and they escape with nothing worse happening.

However, the second time is more complicated.  It is unclear what exactly happened, but it seems that after the group's trip to Lollapalooza, or else after David made his solo trip to that same event, the captain of the basketball team broke his leg.  David traveled back a week to prevent that, successfully, but upon his return finds Adam hospitalized and determines again to fix this.  As he is leaving for this solo trip, Jess enters the basement and sees him departing.  She rushes forward into the field and goes with him.

They arrive on some day in the recent past and she argues with him about his solo trips, but just as she is getting past her upset she--that is, her doppelganger, her younger self--arrives and sees her there.  A feedback loop is created, both versions of Jess flicker, David abruptly returns to the future--but is without Jess, who is now missing and sought by the police, apparently having flickered out of existence entirely.

Significantly, there are events in which the time travelers interact with each other's doppelgangers with no ill effects; it is only when they interact with their own future selves that a problem occurs.  There is no effect when the time traveling Adam touches his past self, only when that past self sees the future version.  On the other hand, when seventeen-year-old David is at his seventh birthday party, seven year old David specifically mentions that he hears him, and it seems probable that he must also see him--in the video from the party he is looking directly at the spot where the older David should be standing at the moment he is there--yet nothing happens.  Thus it cannot be simply that you see your older self; it must be that you recognize him.  It seems that there is this cognitive dissonance, but it is more than merely that you are actively changing your own memory of events (which would be the case both when young David sees old David and when Adam sends Christina to divert his younger self), but somehow when you know you are looking at yourself, and it is your future self.

Let us give kudos for innovation, and then admit that like most ideas about temporal duplicates this one is silly.  There is a danger to meeting your temporal doppelganger--the danger of changing your past self in ways that change who you are and create a sawtooth snap as each change changes the changes.  What happens in Project Almanac, though, probably would not happen.  It certainly seems inconsistent to the degree that it only happens if the younger version of the time traveler recognizes his older time traveling self, and oddly it only happened to people who already knew they could time travel and so should have recognized the possibility that they might meet themselves.  It would have been more of a surprise for the older self, who did not remember this having happened, than for the younger self, who has no reason to be surprised.

They do something like this several times, and they do it fairly well the first time, but then they get shoddy.

The first was pretty straightforward:  Adam had failed an oral examination in chemistry because he had not studied, being preoccupied with the time machine project.  The team traveled back to the day of the test, Christina caught the other, younger, Adam in the hall and told him class was cancelled, and then the time traveling Adam took his place.  It seems reasonably straightforward--but it is not.

The obvious problem that arises immediately is that the younger Adam never took the test, and if the older Adam passed it the younger Adam does not know why he did not fail it or have to take it later.  Thus he will have no reason to make the trip to take the test, and we have an infinity loop, in one history Adam failing the test and so making the trip to take it, and in the other Adam having discovered that he passed a test he never took and never making the trip.

Of course, Adam is smart.  We might allow that he is bothered by having passed a test he never took.  Given that, he would probably conclude that he traveled back in time and replaced himself, and so make the trip.  That is good as far as it goes--but it does not go far enough.  This Adam, who never failed the test, does not know the question.  He would have to find out from someone what he was asked, and have the answer ready; otherwise, the outcome will be different.

The outcome is different anyway, though, because the replacement Adam also fails the test, having made a faulty assumption:  in the original history he got the first question wrong and so failed the test, but when he went back and got it right there was then a second question, he got that one wrong, lost his temper, and failed the test.

Because of this, the team makes another trip to "redo the redo"; but this time the second question is different, and he fails again, so they make yet another trip to the past to replace the replacement replacement Adam with another replacement.  At this point, we have some serious complications.

Let us suppose that Monday at one they traveled back to Friday at nine, diverted the younger Adam and sent in the replacement to fail the test in his stead; we will call this team one.  They return to Monday, Adam checks his books, and Monday at two they again travel back to Friday at nine, team two.  Since he fails again, they return to Monday, he studies more, and they leap back from Monday at three to Friday at nine, team three.  That means in addition to their selves who actually belong in this time, there are three other versions of each of them.  The film fails to deal with the issue of how team two prevented team one Adam from replacing the original Adam, or how team three managed to replace team two Adam.  Note that the same ruse probably would not work:  if team two Christina tells team one Adam that class has been cancelled, he will think she is team one Christina and has confused him with the original Adam.  Further, to avoid encountering themselves, they will have to find a way to divert the entirety of the previous teams from their position outside the science classroom--not to mention figuring out how to avoid trouble for having the dozen of them (three of each of the four not including Adam) wandering the halls during class.

That is only half the problem.  The other half is that the same issue that arises when team one prevents original Adam from taking the test happens again for team two and team three:  once they prevent the team one and team two Adams from taking the test, they erase any knowledge that those versions of Adam ever failed, or what question was asked that caused the failure.  Team two erases the knowledge of the question answered by team one, and team three erases the knowledge of the quesiton answered by team two.  It cannot be done.

The film has a solution to this, but it is a very dissatisfying one, which we will examine next.

There is a quirk in the story of team two.  We easily grasp how when team one Adam answers the first question he is surprised that there is a second.  The problem is that when team two Adam answers the first question and is ready to answer the second, the teacher changes the question.  There is no obvious reason for that to happen.  Yet as we saw the movie thrives on butterfly effects.  We may reasonably assume that the attitude or demeanor of this replacement caused the teacher to pick a different second question.  It is at least plausible, consistent with the notion that the past changes in response to the actions of the time traveler, including in ways he cannot predict or anticipate.

When Adam replaces himself to take his test, there were additional complications involved.  There should have been three teams of time travelers moving through the halls of the school, but somehow there was only one--and this is something that the film does fairly consistently but which does not appear to have any logical basis:  if a time traveler travels to the same coordinates twice, he replaces the previous time traveling version of himself.  Thus when team two arrived at the high school, team one vanished as if they were never there, and team three likewise caused the elimination of team two.

This seriously complicates the problems we just noticed.  Not only do the original Adam and the Adams from teams one and two not hear the questions that the team three Adam has to answer, part of Adam's life has been erased completely.  That is, we have a sequence something like this:  Adam fails the test; Adam travels back and prevents himself from taking the test, and then attempts to answer the questions his younger self has never heard; Adam travels back and erases the version of himself who took the test such that he not only never took the test, he never existed and will never fail the test and make the trip back to retake it; then Adam fails the test again, and again travels back to erase the version of himself who failed the test such that he, too, will never have failed the test.  It would seem then that Adam must have failed the test in the original history and then replaced that history with the fourth attempt in which he passes it--but even though Adam replaced himself in the past, it does not seem that he replaced himself in the future.  Thus we ought to have Adam leaving Monday at one, then finding himself back getting an answer to a question he was never asked and leaving again at two, then again finding himself back getting an answer to a question he was never asked, and then leaving again at three.  He has erased from his own life the events that lead to his actions.

The film took the notion of interacting with your doppelganger very seriously, too seriously; but then they step into a very serious complication using the replacement rules from The Butterfly Effect franchise (which were incomprehensibly disastrous in those films), applying only when the time traveler replaces a time traveling version of himself.  That is a far more serious interaction with a past version of yourself than merely letting him see you, but the film glosses it without even attempting to provide a suggestion either for why it would work that way or for why the time travelers would expect it to do so.  It was poorly considered, and a major problem with its time travel concepts.

Winning the lottery by using time travel is not a new idea, of course.  In The Time Traveler's Wife it was done because the time traveler randomly leapt to the future as well as the past, so he could get the winning number and bring it back to the present; similarly in The Last Mimzy the assumption was that Larry's vision of lottery numbers was a vision from the future.  This time it is done in what we might think is the more ordinary way:  the time travelers watch the lottery results and then travel back a day to buy the ticket.  Well done, and it makes them all if not rich beyond the dreams of avarice at least wealthy enough to pay for school and buy some very expensive presents for themselves and others.

I am not an expert on the lottery; I consider it to be primarily a voluntary tax system which more heavily targets the poor and the elderly (seriously, do you think Donald Trump buys lottery tickets?).  However, I checked the Georgia state lottery web site and found several games available, but none that seem to fit what we know of this one.  The Jumbo Bucks Lotto is the only straight six-ball drawing, but it appears to pay only one million dollars on a six-number match, and only a few hundred for five numbers.  The Powerball appears to have a two million dollar jackpot for five numbers, with a sixth number and a chance to match a multiplier; the multiplier only matters if you do not match all six numbers, and the powerball is independent of the other five, and while the jackpot looks like it could be the fifty-three million the five-number match always pays one million, two with the "power play".  There is also a Mega Millions game, which is five balls plus one plus a spare "megaplier", which stood well above fifty-three million when I looked, but which pays a few million for second place.  That might be what they won, but it seems to be a seven-ball game.

The silly thing is, they got the number wrong.  They were hasty and careless, and their method had room for significant error:  one of them wrote down the number in the future, and another used it to buy the ticket, and the result was that the buyer could not read the other's handwriting and read one of the numbers incorrectly.  Thus instead of winning fifty-three (point-eight-seven-six) million dollars, they won one-point-eight million dollars--three hundred sixty thousand apiece, instead of over ten and three quarter million.  Still, it is enough money to make their lives very different.

They entertain and reject the notion of correcting their mistake by traveling back to buy the right ticket.  Had they done so, they would have had two winning tickets, one for the jackpot and one for the smaller prize--or would they?  This focuses us back on that replacement problem we discussed last time:  if the time travelers traveled back to yesterday to buy a ticket, and now they travel back to yesterday again to buy the right ticket, did they buy two tickets, or did they correct which ticket they bought?  Did they spend a dollar on the wrong ticket and a different dollar on the right one, and does the lottery department have both dollars, or did they spend the same dollar twice?

It seems rather certain that the second trip back should result in a second team spending a second dollar to buy a second ticket; this is inconsistent, though, with the team that replaced itself so Adam could retake his test, and with the solo trip David made to Lollapalooza.  Whenever they travel back to the same time again, or to a time in which they are already present as time travelers, they replace themselves.  Presumably, then, if they arrived in the past to buy the correct ticket at the same time that they were already present buying the incorrect one, they would replace themselves and buy the correct ticket instead of the incorrect one, and presumably they would use the same dollar to do it.  What, though, if they decided they wanted both tickets?  It would not be entirely improbable for someone who bets his favorite numbers to have two tickets, one of which has five numbers correct and the other six.  Would they be able to do it if they were careful to target a time well before or well after their original visit?  Would it be sufficient for them to target a different place--buy the second ticket in Savannah rather than Atlanta--or would the fact that they were already duplicated in one place at that time prevent them from being duplicated also somewhere else at the same time?  How did they get their dollar back?--was it in their pocket when they returned from the past, or did it reappear when they leapt back to the same time?  Are they then the same people they were when they made the first trip, or when they made the second?

They did not attempt it; perhaps the scriptwriters themselves were unclear what would happen and did not want to venture there, but that they considered it should cause us to recognize the flaw.

The trip back to the summer concert Lollapalooza was clever and well done, and should have been reasonably safe:  five more teenagers at a festival among an approximate one and a half hundred thousand people is unlikely to have any significant impact on the event.  The film gets extra credit here for the idea that they bought used backstage passes on E-Bay months after the event and took them back with them.

However, the trip David Raskin makes to change events after they get back repeats most of the issues we considered with the trip to let Adam retake his test.  By changing what he did, he erased the video so he would never see it, and so he would not know to change what he did.  Yes, we have another infinity loop.  Add that to the question of what happened to that David when the other David replaced him, and to yet another problem.

Our second time traveling David returns to find himself naked in his bedroom with Jess, thanks to his intervention, but he has no memory of the events since the first return from Lollapalooza.  What happened to the David who just had sex with Jess?  Where is he, or where are his memories?  Is this his body, or did the replacement David bring his own body with him and somehow dispose of the other?  This memory problem plagues many time travel films--we saw it in Back to the Future--and good solutions are thin on the ground.

The timing here also seems to be complicated.  He returns to find that it is morning and Jess is naked in his bedroom.  We deduce that coming back from Lollapalooza she did not go home, and it is the next morning.  Looking at the sequence of events, they met under the bleachers at school and were gone for under a minute, enjoying nine hours at the concert (and returning deprived of a few pieces of clothing)  We then see them in school (with their clothes); Jess is withdrawn and David is distressed.  He watches the video and determines where he made his mistake, then goes back to fix it.

So maybe it is not the next day that he leaves; maybe it is the same night.  It is certainly nowhere near sunrise.  He returns to a daylit bedroom.  This return time is either too soon (because he has not yet left for his second trip) or too late (because he should have returned moments after his late night departure).  Always the return trip brings the travelers back moments after they left; this time it is different.  Is it because if you change things drastically enough, you also change the time of your return?  Why, then, did he not return sooner, when Jess was still in bed with him?

The problem recurs later, although in a different context; we will have to consider it again.

It is at this point in the story that we have that plane crash.  We saw the chain; the trigger event is that somehow, because David, Adam, Quinn, Christina, and Jess went to Lollapalooza last summer and were gone from the future for under a minute one afternoon in the spring, the captain of the basketball team broke his leg at a party one night that week.  It is the sort of butterfly effect consequence that we see throughout the film, and in fairness they are not always undesirable:  after the Lollapalooza trip Christina is popular with the in crowd, mom has a job, and Adam's video went viral.  It also is unclear whether this was the result of the team's trip to Lollapalooza or of David's subsequent solo trip to fix his relationship with Jess; we learn of it after the second trip, but cannot tell when the change occurred.  We accept that it is an unpredictable unanticipated effect of one of those trips.  The problem is that as David attempts to fix it, we start to have the problems we saw in The Butterfly Effect, that somehow every effort to make one thing better makes everything else worse.

David does prevent the broken leg, tackling the basketball star to knock him out of the path of an oncoming car.  He returns to find that the headline is now that the pilot--a different pilot--safely landed a stricken plane.  He also finds, though, that something happened to Adam, who is now hospitalized and unconscious.  This seems a severe improbability, and there is no explanation given for it, but in the same way that the broken leg of the basketball player could have been the unpredictable unanticipated consequence of the trip to Lollapalooza, whatever happened to Adam could be the unpredictable unanticipated consequence of David tackling the basketball player.

David does not appear even to think about his options at this point.  He immediately returns to the basement and sets the time machine so he can try to save Adam.  Jess enters, sees what he is doing, and (as Adam did before David's trip to deal with the basketball player) gets upset--only more so as she realizes that he changed the critical moment of their relationship to bring them together when they were about to fall apart, and she starts to feel manipulated.

Unfortunately, they are having this argument in the past, near David's house, and unexpectedly Jess--the younger Jess for whom this is the first time through--hears them and comes to see what is happening.  As we saw before with Adam drawing on his own neck, as soon as the younger Jess sees her older future self, something happens:  both of her begin to flicker.

David apparently attempts to avert disaster by leaping to the future, but he is too late:  Jess has vanished apparently several days ago, and at this moment the police are at his door demanding to see him, presumably to question him and possibly to arrest him in connection with her disappearance.

Meanwhile, there is nothing wrong with Adam, and Quinn does not understand why David is asking.  This is bothersome.  The situation with Adam is particularly delicate temporally; all we know is that in the original history Justin Kelly and Adam Goldberg were both fine, but after the Lollapalooza trip Justin broke his leg, and when David made a trip to push Justin out of the path of an oncoming car to put the team back in the playoffs it somehow triggered events which put Adam in a coma.  What David wants to do is break the chain of unpredicted events between saving Justin and harming Adam, but he does not know what that chain is.  The simplest solution, of course, is to prevent himself from saving Justin--but it is not clear that he can do that, if once you have changed the past it remains changed.  That, though, means that Justin breaks his leg and the plane crashes.  Yet the only explanations for Adam's improved condition are either that by making that very brief trip to the past David undid the actions of his previous trip to the past (that is, he traveled to the same time and so relocated the time traveling version of himself), or the disappearance of Jessie Pierce changed history such that Adam did not suffer whatever it is that harmed him.

Next we will explore the problem of the disappearance of Jessie Pierce.

What happened to Jessie Pierce?

When Jessie encountered herself she flickered, and then was missing when David returned to the future.  This seems to be what the filmmakers think would happen if you met your temporal duplicate, although they never explain what exactly is happening or why.

It is somewhat reminiscent of TimeCop, in which contact between the time traveler and his younger self resulted in the two fusing into a protoplasmic mass and then dissolving.  We observed there that the notion of the same matter coming into contact with itself was complete nonsense, and mercifully this film does not suggest anything like that.  In fact, there were no ill effects from Adam touching himself; the problem was created when the younger Adam saw the older Adam, and was described as some kind of "feedback loop".  With that much--or little--information, we attempt to make sense of what is happening.

It cannot be the simple shock of seeing yourself.  If it were, then it ought to impact either version who sees the other, but it only happens when they both see each other, although we have no example of the time traveler being seen by his doppelganger whom he does not see.  Besides, Adam and Jess, the only two persons for whom this happens, are already aware that they are time travelers and might encounter future versions of themselves.  It should not shock them that they do.

The best bet is that the writers are going for some kind of cognitive loop:  it is not physical contact but psychological recognition of the future self that is the trigger.  Let us understand that the older time traveling Jess has, somewhere in her mind, memories of what she was doing at this moment that did not include seeing herself.  Suddenly her younger version living through this time sees her, and now those memories are altered.  Of course, the time travelers are in some way altering the memories of their younger selves relative to their own whenever they do anything in the past--when Christina diverts Adam from the classroom so he won't take the test Adam's memories ought to change.  This might be different, though, because the memories are being altered on an instant by instant basis:  for every second that the younger version of the time traveler sees the older version, the memories are being rewritten to include this.

The first problem is that we have fairly well established that changes to the experience of the younger version of a time traveler do not have any impact on the memories of the time traveler himself in this film.  Everyone remembers the basketball team going to the playoffs; David is disoriented when he finds Jess in his bedroom.  The time travelers are changing the memories of their counterparts constantly, and it never affects their own memories even after they return.  This is the same inconsistency we saw in Looper, that some effects the time traveler has on himself immediately change the time traveler while others never do.  If the time traveler never remembers the history he created, then he does not get the memories of his younger counterpart, and this concept of those memories changing cannot work.

Besides, if it did work that way, it should have no impact on the younger version of the time traveler, and certainly should not cause the older version to cease to exist.  We can see it burning out the brain of the time traveler, reducing him to a vegetative state, but nothing is "changing" in the younger version's mind that does not change constantly anyway:  he is getting his first memories of events, not replacing memories he never had.  Further, if the problem is that the original memories are being changed, that happens whenever the time travelers impact events which impact their younger selves:  when Adam was diverted from the classroom, he thereafter was experiencing a completely different set of events which formed completely different memories of that day, probably with repercussions in the following days as he wonders why he did not fail a test he never took.

Further, as we previously noted, there is a moment in the video of the seventh birthday party when seven-year-old David is looking directly at seventeen-year-old David; why does it not happen then?  The film fails on this point.  The idea is interesting, but it is not credible.

The film is to a significant degree built around seventeen year old David's seventh birthday party.  He sees the video of it at the beginning, in which he sees his older self reflected in a mirror in the background, and ultimately he goes to that party to take the steps necessary to save Jess and himself.  Despite how short this scene is, it creates two distinct major problems for us.

The first problem pertains to David's solution.

Someone once said to me that the problem with suicide is that it is a permanent solution to a temporary problem; that is in part the feeling I have about this.  David has decided that their use of the time machine to change history has resulted in terrible disasters, and concluded that the best solution is to make it so that they never used the time machine at all.  To accomplish this, he intends to travel to that party and destroy the plans for the time machine just before his father's death, so there would be nothing for them to build.

It is obvious that this creates an infinity loop:  if the time travel machine is built, David will travel to the past to prevent it from being built; if he prevents it, he will never make the trip to the past and so it will be built.  What is less obvious is whether destroying the time machine will have the effect the filmmakers want it to.

It is evident that much of the time travel in this movie is based on the popular misconception of Niven's Law:  that once a time traveler has altered events in the past, they remain altered even if the alteration prevents the time traveler from making the trip to the past.  This is a supposed defense against the infinity loops of the grandfather paradox sort:  if you travel to the past and kill your own grandfather, he is dead and you are never born, but you still arrive in the past and kill your grandfather.  This movie is replete with trips made to the past that have made changes to the world--the missing dog, Adam's test, Christina's popularity, mom's job, mom's unemployment, Adam's viral video, the lottery winning, the plane crash, the broken leg, the plane saved, Adam's coma, Jessie's disappearance.  Niven's law (as so understood) suggests that once the time traveler has made that change, preventing the time traveler from leaving the future to make the change does not undo the change.  If it is true on the smaller scale, that Adam does not have to travel back to pass his test once he has traveled back and passed his test, then it must be true on the grand scale, that preventing the invention of the time machine does not undo all the changes the time travelers made using it.

It is of course a flawed rule, as we have previously noted (and not, we think, what Niven meant).  I do not mean that I think destroying the time machine would not undo all that was done using the time machine; on the contrary, I think it would--and that includes undoing the destruction of the time machine.  I only say that you cannot have it both ways:  preventing the time traveler from making a trip which changed the past either always or never undoes the changes made.  That means either David did something completely stupid and ineffectual and will return to a future in which Jess is still missing and the police are chasing him but he has no time machine, or time has come to an end as the world is trapped in this infinity loop between two versions of history.

Yet there is another problem with this final scene, regarding the video of David in the mirror.

In the beginning of the story, David Raskin saw himself in a video from his seventh birthday party, as he is now at seventeen, reflected in a mirror in the background.  He notices several things about this image, including that his one hand is holding an unfamiliar keychain, and his other hand is reaching for the basement light switch.  Having seen this, he explores the basement, has someone switch the switch, and discovers the plans for the time machine.

At the end of the movie, David arrives from the future at his seventh birthday party, and poses for that image of himself in the video.  He then goes downstairs to the basement and destroys all record of the time machine so that he will never be able to find it.

Do you see the problem?

We can blame the problem on confusion of tropes.  It would make perfect sense for David to learn about the location of the time machine from a video of himself having used it to reach the past if this were a fixed time story.  It would be a fairly standard predestination paradox, and advocates of that theory would argue that of course he could see himself in the mirror because you cannot change the past.  However, that is precisely what he is about to do:  he is about to follow the predestination paradox with the grandfather paradox, changing history in a way that prevents him from changing history.  So there has to be an explanation for why he is in the mirror--and note that there is much about his appearance in the mirror that is significant.  It is staged, quite specifically.  He has determined from the video exactly where he needs to stand to be seen in the mirror in the video.  He is moving one hand to call attention to the light switch so that in the future he will know to check the basement.  He is holding Jessie's keys in his hand so that he will find her and know that she is part of this.  The video image can exist.

The problem is, the only reason we have for David being here, in this place at this time, is that he is on his way to the basement to destroy the time machine--and he succeeds in doing so.

We noted that the act of destroying the time machine creates an infinity loop, two distinct histories each of which causes the other.  In one of those histories, David finds his father's time machine, builds it, and ultimately leaves for the past; in the other, he arrives in the past and destroys the time machine so that he will never find it, with the result that he will never leave for the past.  There is no reason why David could not be in this video; but three things are absolutely certain.  The first is, if he sees himself in the video, he will find no time machine plans in the basement, because in the same history that he appears in the video he destroys the time machine plans.  The second is, if there are plans for a time machine in the basement, the video will not give any hint of this; David might see the video of his seventh birthday, but if he has not destroyed the time machine in this history then he was not at the party and will not appear in the video.  The third is, he cannot possibly pose for that image--there being no history in which he both saw himself in the video and left for the past to be in it, he will never have examined the camera angles and so will not be aware that he is going to pass through the video via his reflection in that mirror.  Therefore nothing he does while in frame can be done so that his future self will see it--and yet it seemed to us that this is the probable reason for everything he does.  Besides, since he arrived with the specific intention of destroying the time machine so that he will never find it and use it, there is absolutely no logic to him trying to call his own attention to where it is.

So the biggest disappointment of the film is that it creates this impossibility at the climax.  It was otherwise a very enjoyable film, but it does not withstand even modest scrutiny.  It is difficult to fathom how anyone would have created a time travel movie with such obvious flaws, as even a little thought should have alerted them to the problems in the script.

There are still a few problems left in the denoument.

The ending of the film confuses in several ways, and raises the question of whether the filmmakers ever really had a theory of how time travel was supposed to work.

After David destroys the plans for the time machine, he returns to the future.  However, every other time he returned to the future he returned to within seconds of his departure for the past, but for the one problem we noted when he changed what he did at Lollapalooza.  It happened with the first test and the first human trip, and they make a point of the fact (gone for forty-one seconds on a nine-hour visit to the past) after the trip to Lollapalooza.  It is also clear that the time travelers remember the past they erased, and never the past that actually occurred in the new history--underscored by David's first solo return to find a naked Jess in his bedroom.  However, on this last return to the future, David arrives back near the beginning of the film, several months prior to his departure, in the scene in the attic where he discovered the old birthday video.

Seeking a theory for why it would be different this time, someone probably thought something like this:  because David destroyed the time machine, he changed the past such that he never found the time machine design and so never made even the first trip to the past.  Because he never made that first trip to the past, none of the changes he ever made to history occurred, and so time reverts to the moment he made the first discovery that would lead to the creation of the machine.  In fairness, it might be that time is not reverting to anything, but that the filmmakers mean that all of history unfolded as it did originally up until the moment David was in the attic (never mind that on his seventh birthday there was an unexplained fire in the basement from which there are somehow no butterfly effects) and that history is going to unfold from here--but they do not play it that way, as we will see.

Besides, if the theory is that having undone the trips to the past which made the changes causes time to revert to a moment before the changes, that should have been the outcome of every solo trip David made:  in changing specific events in the past, he undid those events and undid his own decision to go there.  Preventing a trip to the past by destroying the time machine is not different in theory from preventing it by any other means.

Perhaps, then, the theory is that having undone the future existence of the time machine, David has undone his ability to return from the past, but rather than conclude that the older David is now stuck in the past they conclude that he never got there--but the moment to which he returns in the future seems arbitrary, and two points seem to suggest otherwise.

The first is that it appears this time that Christina finds the video camera that David took with him to the past; it perhaps was left in the basement when he returned to the future, and moved to the attic when his mother cleaned a bit.  If David cannot have been left in the past, though, neither can the camera.

The second, though, is that in this history he immediately approaches Jess at school, and makes the somewhat cryptic comment, "This is going to sound crazy, but I think we're about to change the world."  Either he has watched the video and seen the future that has been undone, or he remembers it--and either way, it means we have residue from an erased history, events recorded (whether digitally or organically) that never happened.  This David cannot have been changed by events that never happened, nor can he have any memories of them, nor can any record of them exist.

It also raises questions about what he does and does not remember--and most particularly, whether he remembers that winning lottery number--or indeed, if it is on the video he received from the future.  If so, though, we have another infinity loop, since in this version of history he will never take that information to the past, so he will never have it.  Still, the film has played fast and loose with too many of its time travel concepts for us to begin to guess what would be consistent with its rules.  Despite it being an enjoyable movie with a very promising premise, it comes to a very dissatisfying end despite the suggestion that he will manage to start that relationship with Jess.