Can anyone tell me what constitutes an “incorrectly dated ballot”?

For example, one member of my family started to write out the month on the envelope, then crossed it out and put the month number instead.

I took it to the local election office asking if it needed to be redone, and they said that as long as it has any kind of date, it’s Ok.

Or will some “election observer” look at an envelope like that and say it’s “incorrectly dated” ???