If it's your first 60 day covid extension, it will start from the day you apply. If your visa expires on the 21st, I'd wait until the 20th or so to apply for the extension.
Historically you couldn't because you needed to mail your passport to the embassy. Even if you could mail it internationally to a friend or agent, they'd see in the passport whatever country you're in.
Now with the Evisa though, the Thai embassy has no way of seeing what country you're currently in. The visa is emailed to you.