12/12 Switcharoo Input Requested/Appreciated

Ok, here's the deal. It'll take 2 secs to answer and I am very much appreciative.

I am attempting my first INDOOR grow. I'm scrogging them, pics in my journal/link in signature. My screen is about 60% full and I wanna switch to 12/12 BUT my sunrise is set to 9pm right that is lights out at 5pm-9pm. But I want it to be 7pm-7am when the lights are on. SO, sunrise would be 7pm. What should I do to get this achieved? Just leave the lights on/off for 24hrs prior or just jump right into the 12/12 by setting the timer and leaving em in dark. It wont matter much I know since I want them to flower anyways but just curious how you would do it and what the side effects would be. I know it's only 2 hrs diff if I keep it this way but I leave for work at 8 am and I would like the shit to off while I'm away in case something happens with the filter or something and stinks up the neighborhood. Kinda in the country but still no need to take unnecessary risks right?

I actually thought at the beginning of my flowering that 24 hours of darkness induced plants to flower quicker. I had believed I had read this in Cervantes' Grow Bible but when I tried to find the same info later I could not find it. That being said I don't think the difference between 24 hours darkness/light prior to flowering is going to make a noticeable difference. If it were me I'd go with 24 hours light prior to the switch.