It's a light, fun bit of goofiness with five members of a university SF club and a time machine. (They don't have the latter at first. It only shows up about a quarter of the way into the film.) There aren't really any stakes, except for the hypothetical and never-confirmed risk of unhappening their own causality if they can't sort out their air-conditioner's remote control. Our heroes don't really understand that danger, but they can be persuaded to take it seriously.
Sometimes. When they're not spying on themselves, stealing their own belongings and generally goofing off.
The film starts with a very bad game of baseball. (It's three-on-three, if you include the dog. Who doesn't participate, but still probably isn't the worst baseball player of them.) These scenes are light and apparently pointless, but rewatching later shows that they're jam-packed with time-travelling background details from later in the film.
Our nerds are likeable. They're great friends, full of high spirits and get excited about the same stuff. They immediately understand the implications of a time machine and can short-circuit the usual discussions, e.g. everyone immediately agreeing that the future's a bad idea. Frankly, they're carrying the film with their personalities. The plot's intricate enough to need diagrams on a whiteboard, but it also doesn't really matter. Friends Goof Off. (While travelling back to yesterday.) That's it, really.
Does it make sense? Mmmmm... yes, basically, but it has a couple of head-scratchers. Was that really an accurate portrayal of 99 scant years ago? And not, say, the medieval era? Or, alternatively, does it really make sense that SPOILER wouldn't recognise his own SPOILER? (I'd been wondering if the film would go there, since it appeared to be setting up that twist and yet that also had an obvious plausibility hole.)
You couldn't possibly call this film unmissable, but it's fun. I like the nerd banter. I like the gang's quirkiness, with enough odd and interesting faces that it's almost out-of-place to have a couple of handsome boys in there too. I liked the SF movie buff in a Star Trek shirt.
"You've found yourself a great film! It's a sci-fi B-movie. Two office workers are kidnapped by aliens and sit there waiting to see what's going to happen."
"A real B-movie."
"A C-movie, even. The great thing about it is that it's boring. Really boring. Great, huh?"