@Ralph *******
You are confusing the 90-days VISA VALIDITY with the STAY PERMIT you receive when entering Thailand on this visa.
With a single entry tourist visa, the 90-days visa validity starts on the day of issue
When you enter on this visa, you will get stamped in for a stay permit of 60 days. The visa becomes invalid upon entry.
You can extend these 60-days stay permit for further 30 days on Immigration for 1900 THB
Hence is why many people think there is a 90-days tourist visa, but this is wrong.
A "visa" is used for entering a country. After the entry, you are in Thailand on a "admitted stay" and not on a "visa".
Only on multi-entry visa types, you can enter as many times as you want during the visa validity period.