some seed companies will give 12/12 time and others give actual flowering time, in reality you should only use these as a general guide, ie if u want a quick flower or don't mind a long flower.
Also environmental issues come into it, having too hot temps or too low can slow down, humidity, stress, medium (hydro is generally quicker).
so ignore everything the seed companies or attitude say, they are only here to make money, a 35 day flower will sell more than a 120 day flower. Personally I've never had anything ready before day 48-50 of 12/12, so the 35 day flowerer that I've seen are complete bollox and you'll be harvesting immature cannabis. I counted the other day and I've grown out 134 different strains, some the same but by different breeders, and that's not including clones (I keep all my packets, lol).
so get yaself a USB microscope to 100x and you'll know the perfect harvest time, u can use a loupe30x but I find it hard to tell properly with these, being able to get a screenshot is much better. I also harvest plants over a week or so period as 'usually' main colas finish before lowers. Doing this I can insure the whole plant is harvested at pretty much the same stage, or I can even have different highs if I want.