The answer to when Batman/Batman Returns (and BTAS?) take place?
This article from 2011 makes pretty interesting reading...
http://gothamalleys.blogspot.co.uk/2011/11/when-does-batman-take-place.html
The author reasons that Batman takes place in an alternate universe that has merged 1947 and 1989, with Batman Returns (and I guess by implication BTAS) not long after. The comments are worth reading too.
All for a box of chocolates...