NTP

NTP is one of the most famous clock synchronization protocole that allow a computer to synchronize it time with a server, ensuring to always have the proper and exact time.

"Network Time Protocol (NTP) is a networking protocol for clock 
synchronization between computer systems over packet-switched, 
variable-latency data networks.
In operation since before 1985, NTP is one of the oldest Internet 
protocols in use. NTP was originally designed by David L. Mills 
of the University of Delaware, who still develops and maintains 
it with a team of volunteers.
NTP uses the User Datagram Protocol (UDP) on port number 123.".

Source: Wikipedia

We propose you two tutorials and howto for the NTP on CentOS, one for the client and one for the server.

How to install, configure and manage NTP to synchronize the system clock on CentOS Linux (NTP Client tutorial)
How to install and configure a NTP Server on CentOS Linux (NTP Server tutorial)