[Mjyzpzph]

In addition to the 90 days within 180, you are only allowed 3 entrances...so keep that in mind.

(and that is imho the real frustration)

To the best of my knowledge that information is incorrect.

The following is a worked example from the Royal Thai Consulate that I use for my Visas. It explains how the days are counted up and expired over 7 entrances:

Entry Dates in Thailand total days 6-month accumulation:

1st 01 Oct to 22 Oct 22 22 days

2nd 20 Nov to 30 Nov 11 33 days and 01 Dec to 11 Dec 11 44 days

3rd 15 Jan to 31 Jan 17 61 days and 01 Feb to 05 Feb 5 66 days

4th 12 Mar to 31 Mar 20 86 days and 01 Apr to 09 Apr 9 73 days

5th 07 May to 31 May 25 54 days and 01 Jun to 03 Jun 3 79 days

6th 02 Jul to 30 Jul 29 91 days
in this instance it will be necessary to exit 1 day earlier

7th 03 Sep to 30 Sep 28 94 days
in this instance it will be necessary to exit 4 days earlier

As can be seen with the 6th and 7th entries it would not be possible to stay for the planned full period. It would also not be possible to exit and re-enter without obtaining at least a 60-day tourist visa. The only other alternative would be to fly home earlier than anticipated provided a seat was available.
Failure to exit Thailand by the due date incurs the risk of imprisonment. David