[_OOps_]

I think it's important to be able to work with mod(%) 12 for some operations on year <--> month relationship. All years have 12 months. For day of the month you need additional logic to handle diferent lengths... and other issues.

[zzo38computer]

This reason makes sense. Note that different uses will have different useful conventions, and converting between them may be necessary. (Although this is true of more than only month numbering.)

[jefftk]

This could still work: https://news.ycombinator.com/item?id=32618016