It’s very important to keep your system clock aligned to the exact timezone you reside. Following commands help to keep it in sync.
First select appropriate timezone
# dpkg-reconfigure tzdata
Above command actually copies your zoneinfo file from /usr/share/zoneinfo/ to /etc/localtime
So you can manually copy the file from zoneinfo directory to /etc/localtime on non-debian systems.
Now install following package to sync your system clock with Time Servers using Network Time Protocol (NTP)
# aptitude install ntpdate
# aptitude install openntpd
ntpdate OpenNTPD keeps on checking time periodically to keep system time in sync with time servers.
Thanks poisonbit for pointing out that ntpdate is now unmaintained and NTP or OpenNTPD is preferred over it. Check out first comment.