timer problems

just found out my timer has malfunctioned dont know how long it has died luckily it hasnt shut off currently in vegetive growth cycle and wont be flipping till next weekend what harm am i doing to my plants if i flip from 24-0 to 12-12 or should i do i a little at a time

No harm from just flipping them, or you could run 18/6 until then and save some electric.

Yeah my timers fuck up sometimes aswell, thankfully i don't pay for my timers
however when they fuck up i toss them and replace, and generally it doesn't happen again.
With my timer it has an ''Auto On'' Function so the timer stays on until every off timer and then back on at the obvious on timing.
if yours is the same, perhaps try manually lighting for your needs until you get a new timer?
Early stage of flower you really cant do much damage to the plants aslong as they get 12 hours or more of light they will stay in the veg growth stage, any less than that and they will head into flower.
However the beauty of early flower is that it can be reversed, aslong as you don't have full blown flowers growing.
Sorry for the long post, but i guess its some info to soak up

picked up a mechanical timer today

Sorry did you have a digital timer last time?
forgot to ask, I use slimline digital timers.
They are only about $30 for a twinpack maybe cheaper in your country.