Comment by makeitdouble

> my bank/brokerage statements display dates using it, as do tax/pension forms. It's definitely still in heavy use for some very important data.

I have a hunch the internal system doesn't store it in imperial calendar. While those can be very old system, they went through waves of modernization and in particular handling future payment was always a PITA as you couldn't predict what year it would be 5 or 10 years ahead, or even next year really.

Storing a Unix time and doing the conversion at display time is the sanest way to handle it.

Funny thing is,even with that kind of system the conversion tables can still be outdated, and will cause issue like the ones you had with lenient numbering.