Don’t even attempt to figure out the details of the plot. Just watch it, and take it at face value. The alternative is a trip down a mobius hole that bends back upon itself and leaves you forever falling. You could spend more time reading about the plot than actually watching the movie. So, what does that say?
As for that plot? A couple guys invent a time machine, and use it to go back 1 day and make stock trades. Of course, it’s never that simple, and before long, there are overlapping trips back 2 days, 3… with careful scheming to make everything work just so. All the while, the protagonists are startlingly un-confused… and in fact, not terribly interested – like a couple of slackers who can’t seem to see beyond their own trivial lives.