“Soviet Union observed February 30 in 1930 and 1931.”
So, it must’ve been there and then?
I never knew this:
February 30 Was a Real Date
https://www.timeanddate.com/date/february-30.html
Another version of “2 + 2 = 5”, looks like to me.
After Julius Caesar’s reform of the Roman calendar (46 B.C.), February had 29 days in a normal year and 30 days in a leap year. But when Sextilis was renamed Augustus in honor of the emperor Augustus, he took a day from February and added it to August so that his month would be as long as July (named for Julius Caesar).