How is the "attested from" date determined?

I’ve never made a post here. I happened to look up the word “scavenger” and saw that the attested from date for “scavenger hunt” is 1937, but I know it was used in a film from 1936 and a book from 1935. Is the attested from date just when it was recorded in a dictionary or something like that? I’m sure I’m the only person who will ever notice, but there it is.