Adjusting light schedule during flower??

[transcend]

Hi everyone, just a question about flowering light times..

I'm nearly 3 weeks from switching to 12/12, currently lights on at 4pm, off at 4am.
With the nights and mornings getting cooler (southern hemisphere) I'm getting a drop down to mid 60s when the lights go off even with my heater going, and I can only assume that'll get worse as the weather cools further.

I'd like to push forward the times to day being 9pm to 9am (in hindsight I should've done that initially, first grow oversight) so my question is:

Can I extent the dark period an hour/half hour so that over a period of time the day period gets pushed forward and when I'm at the desired time switch the dark back to 12hrs?
I figure at 12/13 it'd take 6 days, or 12/12.5 would take 12 days. I could go down as far as 15mins extension to the dark period as that's the minimum increment my light timer will allow, but that'd take 24 days and the weather might catch up on me by then.

But if there's any risk of them herming I'll avoid the tactic altogether and just spring for a better heater or try to insulate the tent. It's my first grow and I've been lucky with no major issues so far so I'd like to try and keep the momentum going!

Thanks in advance for your knowledge!

[Rwise]

Just change the timer to the new time while the lights are off.

[JohnEmad]

Same as Rwise said. Change the timers when the lights are off. I have done this several times including my last grow with no adverse effects.

[transcend]

Cool thanks guys, that's good to know!

With all the talk that they're way more sensitive and temperamental during flowering vs veg I thought it'd be more complicated than that!