It's actually really messy. But it basically goes like this:
Anything that was unexpired up until April 30, 2020 is now valid until August 31, 2021.
Certificates between May 1, 2020 and December 31, 2020 are valid for 24 months from date of issue.
Certificates issued anytime in 2021 will be valid until December 31, 2022.
It's especially frustrating if you had your certificate issued in March or April of 2020 and you missed out on several extra months. But there have been a few data points in this thread of people getting them extended when you get a nice rep at the Diamond Desk.