Decades from year

This is probably really simple and I am overlooking the answer....but here goes.

I have a list of people with a birth year, I am trying to come up with a way for example if the year was between 1950-1959 in column N, will result 1950 in column O. I have multiple decades from 1920 - now....

Any help would be appreciated.

=floor(year(a1),10) if its a real date like 20/10/1950
if its just 1951,1952 leave out the year()
=floor(a1,10)