Same here. I write it on the calendar just so I have an idea. I've been growing the same strain for awhile now and they're always ready around the same time so I can pretty much know when they'll be ready looking at the date. But of course they're done when they're done. I like to know how many weeks it's been so I can compare what I think works and smokes best. I've chopped at 8, 9, and 10 weeks to experiment.
I've already decided a date for the next chop since my wife and daughter are going to NY for some hippy shit, lol. Gotta do it before they leave since they're my trimmers, lol. It's all good though. That one will be at 9 weeks.
I put the date I flipped on a stake with the plants other info in the pots. Then I lose it down the side of the pot and always get a surprise when fishing it out after chop.