https://help.ubuntu.com/9.04/serverguide/C/NTP.html
Ubuntu comes with ntpdate as standard, and will run it once at boot time to set up your time according to Ubuntu’s NTP server. However, a server’s clock is likely to drift considerably between reboots, so it makes sense to correct the time occasionally. The easiest way to do this is to get cron to run ntpdate every day. With your favourite editor, as root, create a file /etc/cron.daily/ntpdate containing:
ntpdate ntp.ubuntu.comThe file /etc/cron.daily/ntpdate must also be executable.
sudo chmod 755 /etc/cron.daily/ntpdate