Status at Marriott lasts for the remainder of the calendar year you achieved it + the full next calendar year + a 2 month grace period. Or put another way, you get it for the remainder of the calendar year you achieved it +14 months.
So if you reach Platinum status at any point in 2021, it will last until February 2023. Getting it in August or October won't change the expiration date.
I had extended Platinum in 2021 after getting it in 2020... I just hit 50 nights again, but I had Platinum already so it's a bit weird. I assume that means I should have status through 2023?