Here’s something I’ve wanted to know how to do since forever.
Use this trick to find files that have been modified since some arbitrary date:
$ touch -d "13 may 2001 17:54:19" date_marker
$ find . -newer date_marker
To find files created before that date, use the cnewer and negation conditions:
$ find . ! -cnewer date_marker
And to delete them, use the built-in “delete” action, eg:
$ find . ! -cnewer date_marker -delete