By far the easiest is to store time stamps as modification times of dummy files. GNU touch and date commands can set/get these times and perform date calculations. Bash has tests to check whether a file is newer than (-nt) or older than (-ot) another.
For example, to only send a notification if the last notification was more than an hour ago:
touch -d '-1 hour' limit
if [ limit -nt last_notification ]; then
#send notification...
touch last_notification
fi