When is the first day of flowering iyo?

I find it is best to treat the flower time as a guide rather than a rule. It is when one should start regularly watching the trichomes up close then harvest based on what you see there. For example, if the harvest window indicates 8-9 weeks, I'll start watching the trichomes 7 weeks after the flip and would harvest when they appear mostly cloudy, with only a few clear and a few amber. Too many clear and I get a headache and the high doesn't last long, too many amber and I become lethargic and useless. I find this sweet spot is usually in the middle of the harvest window, so using the example, it would likely be at the end of week 8 that I'd harvest. Since I generally veg them for 6-8 weeks, my plants are mature before the flip and are usually growing pistils in several places so I can only count from when I flip the lights. Are you looking at the trichomes to determine whether or not they're ready? If so what do they look like? Accurate harvest timing is largely determined by ones preferred effects.

I count day 1 of flower the day I flip to 12 hours. That's how most breeders list their flowering times also.

I usually start counting as soon as I flip to 12/12 and plan harvest based on the breeders recommendations. I expect about 1-2 weeks stretch before I see "clustered pistils," but sometimes I've seen "signs of pistils" before the light cycle reaches 12/12 (particularly outdoor plants).

Every breeder I’ve worked with is from 12/12 flip date.. not pistils showing

When pistils start showing. Roughly 2-3weeks into 12/12.

being a mostly outdoor grower I got used to counting the flower time from when the plants actually began flowering so I use that method indoors as well reason being some strains can take a few weeks to begin flowering after the flip which will make it look like a longer flowering strain when it really isn't that is my thoughts anyway