To Steve Ryan about your question about 30 days extension in Chiang Mai: you can extend at Chiang Mai immigration 45 days before the Admitted Until date in your passport, not before, not "at any time" (I asked there). As usual, the 30 days are added to this date.
TLDR : Answer Summary
You can extend your visa 30 days at Chiang Mai immigration, starting 45 days before the 'Admitted Until' date in your passport. It's important to note that extensions cannot be made earlier than this timeframe.