I worked for some time in a bank in France.
Some migrants does not know their real birth date, so their civil status can officially contain a date with zeroes, like `1999-00-00`.
On the contrary, the social insurance system for some reason decided to store this case as `1999-13-13`.
I discovered that in a very complex system that temporarily stored data in a SQL Server's `DATETIME` field, which ended up with people having NULL birth dates and made the whole system collapse.
The system is probably still running today by excluding those people from some fraud detection procedures.
In Denmark we use Jan 1st too. That turned out to be a problem because our personal identification numbers (CPR-numbers) contains the birthdate, and suddenly we started running out of them for Jan 1st some years.
The solution was to remove the checksum requirement (it was a weighted modulo 11 checksum), which is still sometimes causing problems with software that tries to verify it using the old rules. But for some reason that was thought to be better than adding another digit, maybe because it "only" requires every piece of software working with CPR-numbers in existence to be updated but not the data storage...
Ah yes personal identification numbers (BSN in Holland) don't 'spark joy' either.
In Holland every resident gets a unique BSN. So you can use it to uniquely identify a person.
Except if you also have to deal with asylum seekers, because they get a slightly different kind of number which will be reused after the asylum seeker is granted/denied residency.
I discovered that in a very complex system that temporarily stored data in a SQL Server's `DATETIME` field, which ended up with people having NULL birth dates and made the whole system collapse.
The system is probably still running today by excluding those people from some fraud detection procedures.