Spoilers!
At home, near the beginning of the time loop, charge your phone long enough so it's charged enough to turn on, then check the text message saying "help". It'll be too late to follow those introductions, so do the next step on the next loop.
It's actually very simple; enter the elevator on the first floor at precisely 11:53. You'll get a password! Make sure to write it down, as it's pretty long.
UPDATE: So either I did something wrong, or the stranger's code wasn't for the computer. It's an ID number for something.
UPDATE 2: Oh, wait, judging from the credits, it IS a code? Not sure how to decipher it, though.