Ive always been told that if you make a mistake with the photo period its much worse to have the light on when it shouldnt be, so if you're not sure just switch the light off for twelve hours, and then start again on the 12/12. Its putting the light on when it shouldnt be that causes shock, not extra dark.
Good advice.

Using digital timers with a battery backup help with these problems too.. if you loose power, the timer will still be on schedule when power comes back. For things like daylight savings time, or if you have a reason to alter the on/off time (summer/winter temps), simply leave the lights off long enough for the new schedule to start (something over 12 hours). Also, for daylight savings time, I leave the cycle as is.. the plants don't seem to care. One other thing I do is run my flower room on for 11:45. This allows me to stagger when the lights (I use 4 timers) come on and stay under 12 hours. Also helps to compensate for time differences if you're using analog timers.

Peace,

Farmer Rich