I am sure this has been covered before, but darned if I can find it.
My last entry is stamped from Dec. 14, 2017, and 13 March 2018.
Normally not a concern as I would depart and return a couple of days before hand. I have adopted this "visa run" strategy as I have a non "O" multi entry, and had been asked for 16,000 baht to process an extension at Jomtien due to an "irregularity" discovered relating to my residency.
Cut to the chase, my GF and I planned a trip leaving on the 12th, so no problem . However, her employer now says no, we cannot spare you on the 12th. The question is does my permission end ON the 13th, or does it expire on the 12th? I know 90 days from 14 Dec, but do not know if it is inclusive of the last day stamped.
Thanks.

You can go on the 13th.

You can leave on the admitted until date on your entry stamp which apparently is March 13th.

1 minute ago, ubonjoe said:

You can leave on the admitted until date on your entry stamp which apparently is March 13th.

Thanks ubonjoe and Phuket Man. Enough PITA getting her a visa, never mind futzing around about mine. 

The 90 days includes the first and last days which makes 13 March correct in this case. I have got into the habit of checking that the date stamped in the passport is correct, as it is your fault if the official makes a mistake. The page at https://www.timeanddate.com/date/dateadd.html is handy when you want to add 89 days to the date you enter. You can use this in advance, so you know the date the official should use.