i count the day they germ. When i see that tiny white tap root start poking out of the seed husk, that is day 1. And that is for all the seeds i'm germing in that run. So if i start germing seeds today and some germ on the 10th, and then some on the 11th, and then still some on the 12th, i will label
all of those as birthdate of the 10th. the day the first seed of that batch run germs is their birthday for data.
Flower count starts day i flip to 12/12. home growers can use a more nuanced approach, but small/medium scale production requires more regimented hardwired scheduling so to speak. This local guy asked me to produce a pound every 2 weeks this last summer and i had to start figuring shit out fast,lol. Day they germ, day you flip to flower is best from what i've found.