Partial explanation Only for those who have seen the movie.
The movie is shown completely linearly from Abe's perspective. (Abe's the blond one.) I'm sure most people can follow the move up until the scene in the garage where Philip and Robert explain to Abe that Rachell's ex boy friend showed up at the Robert's party with a shot gun.
Things get strange at this point because Aaron and Abe are now planning to stop the ex boy friend, an event that from regular time has already happened but we are viewing the movie from Abe's perspective and he hasn't gone to the party yet. Also at the time of the garage scene Abe doesn't yet know that Aaron used the failsafe machine to change the events at the party.
When Abe shows up at the fail safe machine it isn't the one he left running but the one that Arron had taken back in time inside the first fail safe machine. When Abe goes back in time he meets Arron who already knows what Abe was going to say because he's also gone back in time.
Abe knocks his past self out with the NO2 while Arron drugs his own breakfast milk.
At this point they are changing their own past. They don't play the stock market so they aren't rich at the end of the movie.
Aaron's been recording all the conversations that he hears through the ear piece so he's ready the second time that Abe comes to explain that they have built a time machine, only the second time Abe has also come back in time.
We will never know what would have happened at the party if Aaron and Abe hadn't used the time machine to change the events at the party because those events never happen. We do know that Abe tells Rachel's father the venture capitalist about the time machine and he uses it. That's why he has a couple days worth of beard growth.
Aaron goes though the days several times and even meets himself and gets in a fight with himself.
Now all you have to do is watch it another 3 times to confirm what I just wrote.