Using NTP (Network Time Protocol)
NTP will connect to a server to get the atomic time. It can be downloaded from www.ntp.org/downloads.html To get started with NTP simply download it, install it, use the ntpdate command followed by a public time server, and update your hardware clock.
$ntpdate pool.ntp.org
23 Feb 22:43:53 ntpdate[21751]: [...]