I know Thai immigration count the day you arrive as day one on your 90 day visa. My 90 th  day will be 22 December but my flight out of Thailand is at 02:00 the following day. If I pass through immgaration before midnight on the 22 Dcember before I catch my flight 2 hours later will I be classed as overstay or not. Paying the small overstay cost is not the issue its planning my time so as not to be late for my flight.

If you go through immigration before midnight on the 22nd that is date you will be stamped out of the country.

If leaving from either airport in Bangkok you not be fined for one day of overstay.

Thank you for your reply.... I thought it would be OK but I did not know when leaving the airport you would not be charged for one day overstay which is good to know