# Run it once by hand to get it sync'd now:
/usr/sbin/ntpdate 0.us.pool.ntp.org
# Then put it in cron, to run every 30 minutes
cd /etc/cron.d
echo "*/30 * * * * root /usr/sbin/ntpdate 0.us.pool.ntp.org > /var/log/ntpdate.log 2>&1" > ntpdate