Seriously, every little scrap of writing you do, write the date on it. This is a tip my mother gave me when I was in school and I sort of rolled my eyes at her. Years later, I go through my old writing archives and pull my hair out trying to figure out just when I wrote it.
Believe me, you will not regret it: Date everything you write.
I'm personally even at the point where when I am handwriting something (not so much when I am typing) and I leave off for a day and pick it up again, I'll write the date for when I picked it back up in the margins. Not to mention that I keep a writing journal and so write down when I start and finish new projects.
You will not regret it. You might even appreciate it in the future.

