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I did that. It took me seven years, so 'a couple of weeks' is a bit of an understatement.

That's only somewhat true. If you're exposed to sunlight during the day, much brighter than a lit room, it will tend to have an influence on your circadian rhythm. You might be able to sleep at arbitrary offsets but the quality of your sleep will suffer. Though wearing sunglasses when outside during the morning will help you get to sleep later and wearing them outside during the afternoon will help you sleep earlier. And I'm not aware of research showing a problem with shifting your sleep to arbitrary offsets if you're never exposed to sunlight, though that might have other bad effects.

If you are willing to use window blackout blinds and sunlamps on timers then it's not that hard. However, in collage with a roommate that's not necessarily possible.

In practice, it's just a little slightly harder than you say, when you have a roommate who shuts the blinds every morning and keeps a white desk light on at night.