12/12 schedule ?

I have recently purchased an intermatic timer for my hps light. It was from wally world. My light takes about a minute to completely turn on? Do I need to keep that on consideration when setting my time? Also does it have to be exact 12 hours on 12 hours off? My light is going of 5 minutes before 12 hours of it being on, will that cause my plants to herm or reveg??

It Doesnt need to be exact, 12 off just needs to be the minimum

Lol, no. It does not need to be exactly 12/12 on the dot ^^ My plants are over/under the 12 hour limit by 10-20 mins EVERY DAY. No issues, Its only an issues WHEN THEIR ARE LIGHT LEAKS.. in the MIDDLE OF THEIR DARK PERIOD, or vice versa

It doesn't come down to minutes. The dark cycle is basically mimicking fall causing a response in the plant to fruit before winter. In nature, it doesn't exactly go to 12/12 one day. 12 hours of dark is really what triggers flowering and if you were to skew it one way or another you would want more dark hours to be your error. If you have 12 hours and 5 min of light, you will be fine. What you don't want to do is say have 14 hrs of light once you have started flowering as this will trigger a vegetative response and the plant will try to self-pollinate as it thinks it has missed its chance.

I run 13 off 11 on. Don't ask why, someone told me to do it, and I did (like most time when I didn't know better) but the plants produce and love it.