NTP is a TCP/IP protocol which helps synchronising time over a network. A client requests the current time from a server (identified , and uses it to set its own clock.

The following article on Ubuntu help guide will get you started on setting the NTP protocol
https://help.ubuntu.com/7.10/server/C/NTP.html

List of available timeserver pools which provide reliable NTP service that helps spread the load over many time servers. Check the below link to read about the timeserver pools available worldwide
http://www.pool.ntp.org/

To make changes, edit /etc/ntp.conf

Once you make the changes you can restart ntp by
# /etc/init.d/ntp restart
OR
# /etc/init.d/ntp-server restart

NTP query program

$ ntpq [press ENTER]
ntpq > peer [press ENTER]

You can also use
$ ntpq -p to check if ntp is working