I have done this myself on more than one occasion (Goddam cheap assed timers!!), but most of this is from reading others thoughts on the forums, so although it seems to have worked for me, it may not give you the same results.
My lights are on a 12/12 schedule with timers, the A/C is on a 14/10 and comes on an hour before and stays on for an hour after lights out.
I got up in the middle of the night, and just happened to look at the "Weather Station Monitor" and saw the temps at 98!! WTF? Lights are supposed to be out!! Evidently, the lights didn't go out as planned, but the A/C sure did.
I've been on the forums now long enough to have developed an opinion of who knows what they are saying, and those who parrot info. According to the ones whose opinion I respect, the dark cycle is the important one, and the one that triggers the hormones for flowering, etc. Daytime can get dark early or later, but the dark cycle is sacrosanct. If you have to, let them have a longer night, then turn the lights on as usual. They have to have a minimum of a 12 hr night, It can be 14, 16, even 24 hrs, but the key is UNINTERRUPTED dark If you notice the lights on, say at noon, turn them of and let them go until the next time the lights are scheduled on again so that they'll get a long night. Then return to the normal schedule.
Translation= If your timer screws up, Turn them out and leave them out until they are due to come on again for the next light cycle.
I am the timer now until I can scrape enough together to get a real timer for the entire circuit.
Moral of story, don't cheap out. I have spent more money replacing Yugos than if I had bought the Cadillac in the first place.
The little dual outlet plug in type of timers may well be all well and good for CFLs, floros, and maybe even 400w HIDs, but do not try and use them with 1K lights. It just don't work out well.