I've begun to not worry about a religious adherence to 12/12.

Theres only a handful of days of the year when 12/12 naturally occurs...the rest of the year it can range from 10 1/2 hours, on up to 14+ of day light.

The notion of missing an exact time of lights on/off is furthered when considering...all things being perfect for 12 hours of day light...the travel time for Earth coming around to a plant receiving anything more than ambient light shaves off 3-4 hours of direct light hitting a plant. Or said in another way...the outdoor grower gets 12/12 twice a year, the rest of the time they're either losing and or gaining a minute + each day. I'd think it'd take speaking with a year round farmer in the tropics to realize which part of the year plants either excel or merely get by on available light.

So while maintaining 11/13 - 12/12 - 13/11 - 14/10 is preferred for sake of things being simple...I feel keeping a down to the minute schedule is not a thing that is wholly required in order to maintain an optimal growth rate per unit of energy stored for celluar division during the dark part of the day.

For more info on just how much daylight your burg receives any given day...

http://aa.usno.navy.mil/data/docs/Dur_OneYear.html