Next time you get a 30-day visa upon entry, count the days until you are required to leave - and it will come to 29 days. So, in fact, the policy statement should read; maximum of 87 days within a 180 day period. If you arrive at the airport at 23:55, that next 5 minutes counts as your first day.

The count is 90 days - it is not a multiple of 30 day visa free entries. And yes, day of entry and exit count.

Technically, the stamp is good for exactly 30 days (or 43,200 minutes). You just have to arrive a tiny fraction of a second after midnight and leave a tiny fraction of a second before midnight on the 30th day...

In the real world it doesn't work exactly that way, though. In my experience the preceeding day's stamps aren't shifted until the morning team arrives. I've often arrived at Don Muang around 5 AM and the stamp from the preceding date has never been properly updated

Technically, the stamp is good for exactly 30 days (or 43,200 minutes). You just have to arrive a tiny fraction of a second after midnight and leave a tiny fraction of a second before midnight on the 30th day...

In the real world it doesn't work exactly that way, though. In my experience the preceeding day's stamps aren't shifted until the morning team arrives. I've often arrived at Don Muang around 5 AM and the stamp from the preceding date has never been properly updated

...or 2.592.000 seconds.