Good question... we'll hit 12 by Sept 22... when growing outdoors (people who know) have told me theirs start flowering now (or earlier... but that's when they've already been vegging (usually all summer).
The plants in that case are reacting to the lowering in light as opposed to waiting for a full 12 (12 is just an "optimum" and since indoor lights don't generally get progressively lowered... it's what's best to shoot for)
I'd sort of think you might have issues (early flowering) if you keep it out there... but on the other hand... it also makes sense to me that it will not... vegging out until it does hit closer or past 12/12... I know I can't go the other way: mine (flowering indoors at 12/12) out now... despite the shortening days, it's longer than I've been giving them so they'd likely re-veg or go hermie.
If you can afford to, please try it... if you're vigilant you will start noticing signs of flower too early early and get them in and vegging without having problems (slight slowing/stunting, small chance of stress induced hermies).