When does my 90-day reporting period start if I re-enter Thailand on a retirement visa?

From your new entry date.

Date of entry is day 1, not day zero.

Your 90 day reporting 'count down clock' starts over the day you stamp back into the country and your next 90 day report is due 89 days from the date you stamped in (the day you stamp in is day ONE)

Now as far as if you have to file a new TM30 that is a 'check with your office or post on a group related to where you live and ask' thing

SOME offices say if you're returning to your primary residence from abroad you do not need to file a TM30

But

SOME offices say you DO need to file a new TM30 when ever you return from overseas.