The hint for what is going on occurs at minute 13, with Abe lying on the floor, taking a call from Aaron, is shown doing everything twice. Clearly, a box already exists.
We start getting the story that this movie is about time travel starting at minute 21. Abe is methodical, as evidenced by the way he explains it to Aaron. First, with the Weeble, he introduces Aaron to one scientist, then another. Then he shows Aaron the watches, digital and clockwork.
A hint is what Aaron's first reaction now is, "You're talking about building a bigger one."
It's also a bit amusing that Abe has figured out lots of efficiencies with the design that surprise Aaron.
Abe then brings Aaron to the self storage unit. Quickly we learn Abe has gone into the box, at least once, to go back in time, without letting a word slip to anyone. That is the type of people we are dealing with, here.
A hint is Aaron's first words after the explanation is complete. First he finds out what Aaron did, his first day through, then "Now that I know about this, don't do this again, OK, not where it affects me."
By the end of the movie, we learn Abe had, in addition to the box he showed Aaron, built another box. This is the type of person we are dealing with, here. We actually get a hint of that, too, for the first time both of them go to the storage units to enter the boxes at the B end Abe slows at the wrong locker, and Aaron notices.
Explanation: Just like Abe, whose progress we watch, Aaron's first instinct, the first time he gets the field to work, was to build a bigger one, and hide it from Abe and the others. In effect, Abe ends up doing all the bug testing for Aaron. This is Abe's curse, for being so methodical. He didn't even imagine that Aaron had done the same thing he had, i.e. build another field.
At minute 39, we can guess that the reason the static shock for Aaron is a lot more powerful than it is for Abe is that Aaron is, already, nesting his time travel, while Abe is not. That's more of a guess, but seems just as plausible as Abe's explanation, which was that Aaron got out "too soon."
Even though it is actually months in the box, Aaron goes back to minute 13, with a box inside his box. Why? Because he has killed Abe. In the airport, Abe tells Aaron he can never come back, because he has killed him.