Some start counting the night they switch over to 12/12, calling that day 1.
Others (me) start counting when they see the first actual pistil, usually 7-10 days after going into 12/12.
It's a personal choice.
its a personal choice as stated above. But i start counting when the first real cluster forms on top preflowers mean about 4 to 5 more days before flowering. I have come to the conclusion that that is when seed companys start counting because when i started counting at the first real flowers my afghan was taking about 55 to 60 days. I hope this helps happy growing