Like I said, it is slightly strain dependent, but the flowering period for most plants will be shortly after the nights or dark period have become significantly longer than 10 hours. So it is not that the days are getting shorter that triggers flowering as much as it is when the plant gets significantly less that 14 hours of light and significantly more than 10 hours of uninterrupted darkness, this is what mostly triggers flowering.
And I meant to say you don’t have to wait until the equinox to start counting, lol. I got solstice confused a bit, so you don’t have to wait until the 12/12 hours of light and dark for flowering, it will start a little earlier than that, somewhere a bit after the nights have become at least 10 hours long.