Well, the Breeders and Banks have an interest in having short flowering periods, so they almost certainly be a tad earlier than in practice.
IMO, there's already enough subjectivity in when to harvest that for any accuracy it's best to go by the 12/12 switch. It's a definitive starting point that's always the same for all growers no matter where or what conditions.
Outside is it's own story and doesn't have the control we do indoors. Indoors we're all about control, lol, there's no reason 12/12 isn't the standard for Flowering Time with an Early, Prime, Late Harvest week number.
Don't get me wrong I also note various times for things like pre-flower, pistil color change, trich. color change etc. but those numbers are way more about what my environment is doing, and not the general timeline of the Strain.
I'm probably overthinking it but yea I like the 12/12 start because I know exactly when that happens and don't have to guess if those pistils are the right length to start the flowering clock, lol.