I had a similar issue on my last grow. My timer f*cked up, and suddenly I was a few hours out of whack. I felt since I was already in flower, the plants were triggering their processes from the dark cycle and getting their energy from the light cycle. So, I adjusted only the "lights on" cycle by going backwards. Everyday I went in prior to "lights out", and changed the timer so that the lights would go out one hour earlier, and come on 12 hours after the "new" time.
That way the dark period remained at 12 hours and the "lights on" period would be all that was changed. Mine showed absolutely no signs of stress because of this. Your mileage may differ.