Linux systems can sync with time servers on the Internet using the Network Time Protocol (NTP).  The ntpdate command sets the clock initially, while the ntp daemon periodically monitors the clock skew and adjusts the system time accordingly.

Installing the required packages

If you don't know what system you're on, please see How To Check Your Linux Version

On RedHat Enterprise Linux (RHEL):

sudo up2date install ntp

On CentOS:

sudo yum install ntp

On Ubuntu:

sudo apt-get install ntp ntpdate

Syncing the clock initially

The command to initially sync the clock is:

sudo ntpdate -u pool.ntp.org

You may want to run the command a few times until the reported offset is less than 1.0.  Your clock should now be accurate.

Keeping the clock accurate

The ntp daemon periodically monitors the clock skew and adjusts the system time accordingly. It should be started now, then configured to automatically start when the system boots.

On RedHat or CentOS:

sudo service ntpd start
sudo chkconfig ntpd on

On Ubuntu systems, the ntp daemon is started automatically and is automatically configured to start on boot when the ntp package is installed.