$ sudo ntpdate ntp.ubuntu.com
31 Jan 07:00:03 ntpdate[1970]: adjust time server 91.189.94.4 offset -0.071235 sec
Or ntp daemon which calculates the drift of your system clock and continuously adjusts it, so there are no large corrections.
$ sudo apt-get install ntp
$ sudo service ntp start
* Starting NTP server ntpd
Note: modify /etc/ntp.conf to change default servers.
