Find allows you to find files, e.g.

find by time

find files older than ...

To find files in folder that are older than 5 days use

find folder -ctime +5 -exec ls {} \;

To find how old a file is, use the command stat.

To find files whose content has been modified 1 days or shorter ago use

find . -mtime -1

To find files whose content has been modified 1 days or longer ago use

find . -mtime +1

find -not -empty -iname "foo*"

To find files by content you do not use find, but grep.

find . -name '*.mp3' -exec echo "Do something with the file" {} \;