It's not that hard to just have a concrete storyboard and stick to it, or to go back over the timeline in post-production and make sure you didn't miss something that was written to happen differently at an earlier stage of production, but aside from the basic plot of the girl going missing, nothing holds up. To note, the reason I am giving two stars is that there are elements outside of the story/etc that were really well made.

*spoilers start here*

The whole reason that Will knows that the older lady ends up missing is that the investigator says something along the lines of "Her hair was in the car that we found your prints in" and Will says "I didn't kill Emily" and the detective says "Oh you think we're still talking about Emily?" but then later in the movie there's no point in which Emily is in the red car, however it is justifiable that it was in there anyway given the ending. Unfortunately this doesn't plug the hole, as Will was never in for questioning after the older girl went missing. He immediately realized who took her when he woke up and went to the kidnappers house.
There is this whole thing where for the first three quarters of the movie, they are saying/implying that the little girl is dead and that they know that she is dead, but even accounting for the ending where it's revealed that you're allowed to change the timeline as much as you are able it doesn't make sense to have it one way then flip because it is revealed that the person who kidnapped the little girl was purposefully waiting to mess with Will as much as he could, so that's not something that could have changed.
The cook is shown to have jumped more than three years in the future when he's testing the drugs, but for some reason was still aware of the wavers that he made people sign.
The main character could control his body every time he went to the future, but never when he went to the past... except for one scene where he went back like six times in five minutes and was in control every time.
The costar girl makes a video that the main character watches at the end saying that she had agreed not to take the drugs anymore but then immediately after says that both her and the cook had been taking them regularly throughout the time that she had known him.
They never explain why it was justified to give something like this to random strangers as a street drug, and they weren't actually monitoring the effects or experiences of any of these people, they only stepped in when someone was going to die(or so the end of the movie 'revealed')
The reveal that the monologues you heard "throughout"(read "twice in") the movie were from the videos that she was making to document her experiences after she woke up from the drug(even though not one of the videos did she refer to what happened when she was tripping except for the one where nothing happened as a form of significance) falls through because they didn't put the hospital sounds in the take that was used in the first of the movie. They really could have made it cooler to have the reveal that they were already going around saving people be inside the videos. Even with the rest of the flaws I at least would have liked that ending.
They make a big show of him complaining about being useless if he doesn't sleep, but then it turns out he has "no income"(even though I thought I remembered one of the opening scenes being of him doing work on a computer, I could be wrong on this one)
They kept saying that he was traveling one day into the future, but he had tripped like three or four times before the girl actually got kidnapped in the main timeline. The first scene where the drug-dealer comes over(I'll try to let this be the only time I mention him) Will was just waking up and then shot up right when he left again which was before he had been seeing the future and was only taking the drugs to sleep.
If they could have culled these issues to being things that would have been plausibly variant between timelines then that would have made this a great movie. Time travel is almost never handled correctly in films and making people think they screwed it up until the reveal(which you don't even realize right away explains all of these things, it would be a couple weeks later for some) gives what would have otherwise been holes a new light. This would have been a HUGE nod to the idea that they're aware most film makers can't do this stuff, and don't even try to understand it beyond just being 'cool' or 'weird' or 'incomprehensible so why even try.'
The character continuity doesn't hold up, either. There's no reason for their personalities to change just because we don't know their backstories yet. What would be the benefit of pretending you have borderline manic depressive disorder(formerly known as bipolar)? That's like a really serious issue that someone who's a guinea-pig for drugs wouldn't be allowed to have, especially something that only one other person's tried.
There's no reason that the main character wouldn't have woken up from finding out the girl was missing and just kept watch while he was awake, the entire premise for the movie was that he was sitting at home doing nothing all day, except for the fact that there was this mystery to solve, unless he was asleep. In fact, the very first time he looked out the window it was very obvious that there was a man in a car staring at the little girl.
They made it so that his sister can talk to him from beyond the grave, but is also omniscient when it comes to his situation and then he can hug her after she's dead on a dock in one of his trips where she's aware that she's dead but this has nothing to do with anything and just serves as an example that one of the producers or directors doesn't understand that not everybody assumes things like that are a given in real life. It really didn't seem to be something they put in to further the plot/atmosphere, just that either they ripped the idea from movies that people respected or they actually believe that's how it works. And once again, this only applies to one scene.
Since I have got to go to bed, I'll end with a character time continuity error combination. If the kidnapper was kidnapping the napped kid because the drug user was investigating the kidnapping that he saw in the future, then why did it happen at all? No one assumes that you're dating someone just because they're over at your house one time, and clearly he had been watching Will since before Will met the older female so it should have been obvious that he didn't have a connection with anyone else in the movie except for the kidnapping coming about.

Some of the acting is poor (the detective, for one example), but the plot and pacing were great

Interesting idea and well-plotted. The execution was on the cheap, which may or may not appeal to those who are used to higher production quality in more mainstream movies. The actors had talent but might have benefited from a more experienced director/writer of full feature films. The material treats the viewer as intelligent and leaves a lot for you to figure out, which I enjoyed. One of the benefits of such a production is no need to cater to the low-brow execs of big houses like Sony or WB. Overall well worth the watch.

As with all time travel movies it can be tricky, but all makes sense in the end ... hang in there.

excellent movie. adventure, mystery, suspense. tiny amount of sci fi. not recommended for kids (heavy drug abuse, which is a part of the plot). beginning is a bit slow, but the middle and end are worth every bit of time you spend watching the movie!

meta

One star because it is almost sci-fi, well, here and there --it tries to be...it deals with, or pertains to subjects germane to the science-fiction---it utilized 'sciency,' futuristic-like accounts of society run amok.

Now, Okay, at first, the film's "time-complex-concept" becomes a problem. It muddles the plot, and, the action-at-hand. Simply speaking: hard to follow.

Can not recommend as a Sci-Fi Movie; I would suggest calling it a fictional account of humans messing with a highly addictive, futuristic, psychedelic ? drug, or the so-called "Z" super-psychoactive substance, an experimental drug which exponentially enhances cognitive function and in turn messes with the users, err...addicts, entire 'time-concept-format' and induces PTSD, unlike "Limitless."

We follow this stuff, Z, like the movie, in its mind-numbing, generic content ---ack, ack background noise, tense scenes, thugs, braniacs with interesting dialogue, here and there, relevant to nothing; more background music, chatter, night noises, more chatter; night sounds --- your regular T.V noise, etc.etc., always a must to feel at home.

I would increase the rating if I could get back some, even a little part of my time spent watching, trying to understand this over-rated movie. And how zealous the use of Z, and the zeal behind the little designer molecule responsible for the "Altered" in Hours (and Minds!)
spoiler: "Altered Hours," is not Science Fiction.

This film is another iteration of the inherent problems in designer drugs, addiction, consequences of choice. And this is by no means a harsh criticism of the movie, its reflecrion on current socio-economic, and political times, or the times themselves. I'm not sure what to say about some commetaries as seen in movie reviews, Here, and there ---
Thank You.

If you like complex, quirky time travel stories, this one is worth a watch. At first, it was difficult to understand and I was not too sure about the acting, but as the plot begins to be revealed, the story begins to take shape and the acting improves and the characters and their actions make more sense, though there are twists and turns up until the ending... which is perhaps a little too neat, a little more ambiguity would have made it more interesting to me.

Loosely referencing some ideas and situations found in Philip K. Dick's novels, this movie's overall tone is closer to the spirit of PKD's books than most of of the blockbuster movies that have been more explicitly based on his stories.

All in all, I found it quite fascinating and much better than I expected.