[msla]

> These don't sound like real reasons to me.

They are if you have to write date-handling code or, worse, use the date-handling code of others.

Date-handling code is hard enough without NULLs.

[ketzu]

This does not only concern code, but regulation, tradition, contracts etc. The mutual understanding of weeks, days, months of roughly equal lengths is baked into a lot of our things and it's not enough to change it in some header file/law to clean up the rest.