Find allows you to find files, e.g.

find by time

find files older than...

To find files in folder that have been changed more than 1 day ago use

find folder -ctime 1

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

To find files in folder that have been changed less than 1 day ago use

find folder -ctime -1

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

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" {} \;