I think parent means that we often say:
> I get my check on the first of the month
But never really say
> I'm going on vacation for the sixth (month) of the year
Seems like they are just trying to explain the bad decision, not justify it.
Yes, I'm not trying to justify it. And it's not just that we say "the fifth day of May," because it's also true that we can say "the third month." My point is that the fifth day of May is actually named "5" and there's no other name for dates (other than specially=named days, of course).
1970-01-01T00:00:00 is the start of the Unix Epoch, it is definitely not 1970-00-01T00:00:00.
This is consistent across all the warty and various ways to represent the Gregorian calendar, the fifth month is always May, no exceptions.