The Didache

[rockfox]

Since in that work the visit of an apostle or of a pretended apostle is contemplated as a not improbable event, we cannot place the book later than about 80. The limit, would seem to be from 65 to 80.

I don't care for that line of argument. It is attempting to date the text based on a theological interpretation which may not be true and may not have been held by those who wrote the Didache. Esp. since Orthodox teachings don't interpret 'Apostle' that way.

Just because the requirement to be numbered with the 12 was to have been with Jesus' earthly ministry from the beginning, doesn't mean that was generally true of all Apostles. It wasn't for Paul, probably wasn't for many of those Paul called Apostles.

This thing about Apostles being just the 12+Paul is a Protestant hangup. The Greek just means messenger/ambassador. Protestants today have apostles, they just call them by an alternative word: Missionary.