Subtract two times in Bash

Here is a short script that I made to subtract two times.  It is a little harder to do on a computer then it is in your head.

#!/bin/bash
DATEONE="00:02:20"
DATETWO="00:04:40"
DATE=/bin/date
echo $DATEONE
echo $DATETWO

SECONDSONE=`date -d $DATEONE  +%s`
SECONDSTWO=`date -d $DATETWO  +%s`
diff=$(($SECONDSTWO-$SECONDSONE))
echo "$(($diff / 60)) minutes and $(($diff % 60)) seconds elapsed.""

# ./date
00:02:20
00:04:40
2 minutes and 20 seconds elapsed.