Use the "-newerXY" specification. For instance:
Variants allow you to compare the timestamps of the file to the timestamp of a reference file (find -neweram foo.bar) will find all files accessed ('a') since the modification ('m') of foo.bar .
# finds all file modified since March 20
find . -newermt 20130320 -ls
If you need older files, then use "not newer", you can even combine several tests:
# find all files changed between March 10 and March 20.
find . -newermt 20130310 -a -not -newermt 20130320 -ls