What constitutes a year for multiple entries under the Visa Exempt rule in Thailand?

I understand a tourist can enter Thailand via Air up to 6 time per year on Visa Exempt. I'm retired, purpose of visit - tourist, and I only stay max 30 days, then return home always with a return ticket.

Question is: What is a year? Is it a calendar year? Or is it a year ago since the current date? For example: Now is July... do they go back to July last year to determine a year?

Last time I went in late May, the airline at check-in asked if I had a visa (I didn't) and they looked through my passport, but let me fly.

I've been there 3 times this year, and am planning to go again soon, visa exempt.

Any suggestions?