I count from the day I change to 12/12. Its easier to keep track that way. Plus Im lazy 
Like @thenasty1 said, it doesnt matter when you start, they wont be done until they are done.
On top of that, even if the breeder says they are 7 week flowering or 12 week flowering or what ever, there are always huge variations. Plus, the breeders rarely say when they start counting, so you really have nothing concrete to go by.
Bottom line - do what ever feels right or easiest to you. You could just as easy follow @SeymourGreen suggestion and be just as correct.
You wont be wrong no matter what you do
Edit: remember, most of the “rules” for growing this plant came from stoners. So there is more than a little slop in there 