@Joe ******
it would start on the day of application IF they accept your application.
At most immigration offices, they would accept the application when you have 30days or less on your current stamp.
Bangkok, CM, and a few others allow you to apply for an extension 45days or less left on your stamp.
There were reports that when people could get a 60days Covid extension first, some people had to sign to give up a normal 30days extension.
It means, if the immigration office accepts your application for a Covid extension, there is a possibility that you would be able to stay in Thailand shorter than when you apply for a 30days tourism extension normally because tourism extension will be added to your current admitted stay period (you would have 89days 90nights total).