I and I believe most people count the days after the light has been switched to 12/12. So, when you see seed companies advertising flowering time at 6-8 or 8-10 wks it means typically this strain will be finished in X-XX wks after changing the light period to flower.
I keep a journal along with pics and keep notes on stuff like that ;when I first started seeing flowers, measurements in height, what i put in my nute solution and in what amounts, date I switched to flower, etc. I also jot down ideas and stuff I learn from the farm.
This information is really valuable to have especially if you grow that same strain again.
Good luck