i never flower a plant without preflowers, fully mature. i use mothers from seed, grown and sexed, fully mature. then i take cuttings and root/veg them. then, the switch is the first day of flower, as they are fully mature and have female flowers on every node.
if you flower before the plants are sexually mature, they will continue to veg even in 12/12 until they hit sexual maturity. then, they will flower. so...
if you are flowering a premature plant, your time is not going to be accurate.
if you are flowering a sexually mature plant, the first day after the first dark cycle is day one of flower.
perfectly stated, and i agree, 100%.
i too always wait until my plant is sexually mature before i flower, ie, see preflowers first.
saying that though, like someone else said, the times a breeders gives as to how long their strain will take to flower is a loose guide line at best imo.. too many factors play a role in how long it takes a strain to flower, phenotype being but one small one, and they'll be finished when they're finished.. i like to take note when i start flowering just to give my self an idea of how long they've been flowering, and how long they take, for the next rounds..
lastly, a lot of people think that finished is much different things.. i've seen people say that a plant is ready for chop when it still has a lot of pistils that haven't receded back into the buds, while other people like to wait until they're all gone and the trics are showing a certain percentage of amber or w/e// i've seen people showing pix of bud that they've just chopped and i think to myself, why on earth did you chop them, they look no where near ready.. all personal preferences of course..