@James ********
AI is not the final authority, but this is what it says:
AI Overview
+2
The 90-day period for a Thai visa starts on the day of entry into the country, not the day of approval. You must enter Thailand within the visa's validity period (usually 90 days from the issue date for a single-entry visa), and your 90-day period of stay begins on the day you enter.
Day of Entry: The 90-day count starts once you are physically in Thailand.
Day of Approval: The date the visa is approved or issued is not when your 90-day stay begins. This date is a deadline for you to enter the country.