Originally Posted by Angus

No, the solstice (equal periods of light and dark) is at the same time no matter where in the US you are.

12/12 is used indoors as a harsh contrast to 18/6 to 'snap' the plants into flowering so you don't have to gradually decrease the photoperiod as in nature. Imagine if you had to change your timer by a minute or two every day. It doesn't have anything to do with the sunrise/sunset, it is just an indoor growing technique.