Hi, I switched to 12 hours of light a week ago, My timer seems to be on the blink, the last few days it hasn't gone off on time and my plants are getting 13 hours or 13 and half hours the last few days. My timer keeps failing will this harm my plants. I have got the timer sorted today, so will they be ok.

Have you got a contactor?

If not get one now.
Best luck.
Il P

Yes I do have a contactor the timer is old though got a new one maybe this will help.
I was more concerned about them getting more than 12 hours light for a few days , I was wondering would this harm my flowering cycle even though I have the timer sorted now.

In that case, you are only one week in, it should be fine.

Yeh they'll be fine mate don't worry just get it back to what it was as soon as you can