Disney movies and missing copyright dates

[BwanaBob]

(Maybe I've not seen enough of a sampling, so someone may correct me)

We have a boatload of Disney DVDs, (99% of them are animated flicks).

Wanting to sort them by release year I started looking for copyright year.
(Music CDs always seem to have this somewhere on the disc or jewel case.)

But these DVDs do not have copyright years printed on the discs or the backs of the cases.

Why would they do this?