there are lots of choices for this depending on just what you want to do.
cron if you want it to run on a schedule (i.e. Every day at noon, 48 seconds after every hour, or 11:30 PM every Tuesday and Friday in June, August, and December)
at if you want to schedule it to run once, at a specified time
sleep [seconds] if you want to wait a given number of seconds and execute it at either the command line or in a shell script. (at the command line, you would for instance "sleep 60; echo "One minute elapsed. Shutting down." shutdown -h now" with the semicolon separating your commands.)