First, you'll need rdate installed. Note that rdate uses an older time protocol that is not supported by all NIST servers, but is supported by most.
Just add a cronjob that runs rdate once a day. You can modify this to run every hour as well.
Before you start, find out where rdate is located by running:
which rdate
It's typically installed in /usr/bin or /usr/sbin. Modify the cron job to reflect the correct path.
Here are all the steps.
crontab -e
0 6 * * * /usr/sbin/rdate -s time-c.timefreq.bldrdoc.gov