[schlowmo]

Not so long ago I had to deal with the Austrian version of a Social Security Number and it turns out that using birthdates inside such a number isn't a good idea, since

* Some people doesn't have a birthdate with which the Austrian state agrees

* For those people the Austrian state at least assumes a birthyear with a birthday at January 1st or July 1st

* This leads to overflowing the available numbers for those two dates

* If a date overflows, they get SSN with month "13“. The 13th month can have as many days as needed (so > 31 is possible).

* Now you have SSNs with a month part of e.g 2023-13-32.

Why even bother with birthdates then?