I have just booked a trip to thailand I arrive on the 06/12/15 and am scheduled to leave on the 05/01/16 which is a total of 30 nights but I leave on the 31st day will i need an extension for that 1 day or is it counted by the nights. I will not be getting a visa just the tourist visa exemption. will i be fined for overstaying?

It is counted on actual days, not nights.

A 1 night overstay will be noted in your passport, but not fined when leaving Thailand by air.

Not sure how things will be over a year from now unless you got your years wrong.

You will not be charged for a overstay fine of less than 24 hours if leaving from either airport in Bangkok. So no need for an extension.

Leaving from phuket. 2015-2016

You have prebooked for more than one year? (2015/2016) Really?

Leaving from phuket. 2015-2016

You may be fined 500 baht (unless changed before then) for a one day overstay.

"It is counted on actual days, not nights."

I thought they go by dates, not days or nights.

Day one is the date you arrive, even if it's only a short time before midnight.

A day is 24 hours long. Midnight to Midnight. The day you arrive is day one.

You have prebooked for more than one year? (2015/2016) Really?

I'm sort of surprised an airline would accept a booking over a year away since flight dates, times, costs and such tend to change over relatively short periods and, as hinted above, the rules & regulations about overstay could very well change before then.

Airasia had a promotion with free seats on flights upto Jan 16 2016