The key is the -d parameter for the date command. With it, you can give the date in, say, '21 Dec 2007 12:14:56' format, and it will give you the date with those dates & times, and you can then adjust the format with the usual switches. For example
date -d '21 Oct 2006 02:43:13' +%swill display 1161416593 which is the number of seconds from the epoch to the date/time given.
This script will take a date in various formats and tell you whether it is more than 15 minutes old or not. It's not very useful by itself, but it shows the code that you can adapt for your own purposes.
recsecs=$(date -d "$1" +%s) cutoff=$(date -d "-15 minutes" +%s) if [ $recsecs -lt $cutoff ]; then echo "Too old" else echo "Time is within range" fi