Everyone times it different. My conclusion and what I now do is to start my day count not during transition but when the first bud appears. I have found that my plants harvest then aligns with the breeders claims of time with. What the breeders "flowering time" states!
Flowering also obviously is slightly affected by nutes, environmental factors, etc.
Checking the trich's is the only way to know for sure, and I also go by the pistols fading.
Way I see it, it wouldn't make any sense to say "from start of 12/12" vs "from start of 1st signs of flower", because what if the plant takes 3 weeks to start flowering? Then it'd be like that time isn't significant, when it sure is!
I think if they say something like it's 9 weeks when it's really 11, it's them just bullshitting, trying to advertise the plants as more appealing. These days, seems like the industry has been virtually overtaken by a sea of rookie growers in it only for the money, way I see it, of course there are going to be companies happy to feed them the bullshit they want to hear. Imho.