It's best to have the 12/12 cycle exact, you may be able to get away with a small variation, but it's risky.
If need be go spend $10 at the hardware store on a cheap timer, it's taboo to alot of growers to use a cheap hardware store timer to power your lights, but in many years I've NEVER had a problem with my $6 timer from ace hardware.
No way to speed up the flower cycle, sorry.
peace