It looks late 1930's or very early 1940's to me, also. It's quite long, unless it's a late afternoon or dinner dress. It looks like something an older woman, perhaps a dowager or widow would have worn. I have seen/handled a 1930's piece with a center back zipper but it is unusual. I remember posting about a piece in the UH Historic Costume Collection (when I was doing some volunteer work a few years ago) with a center back zipper. One of our members pointed out that Joan Crawford wore a gown in "The Women" that had one!
Linn