Bob Waltz is to humble to say it, but the Traditional Ballad Index (which Bob edits) is very good at establishing dates for songs. One caveat - the date of a song in the Ballad Index is the earliest date found in the sources indexed by the Ballad Index - it is not the date of composition.

See: http://balladindex.org/

Also see the other thread on "Mary Don't Weep," https://mudcat.org/thread.cfm?threadid=7873