Hi, Alan - people always question the dating system used by the Traditional Ballad Index, but it makes sense. The date is the earliest publication date of books that have been indexed that contain the song in question. The song may be much older, and there may be older books that contain the song, but those books have not yet been indexed by the Traditional Ballad Index.
The date does come in handy, because it sets a solid date for the first known time that the song appeared in print (within the limitations of the indexing process). This can affect a performer's need to pay royalties.
My Cazden book takes it back to 1958, but I'm sure it's much older than that. Apparently, it's also in the Oxford Songbook, published in 1927 - but I don't have access to that book and would be reluctant to swear by any source I can't look at.
-Joe Offer-