How to setup centos as ntp client

# yum install ntp ntpdate ntp-doc
# chkconfig ntpd on

Synchronize the system clock with 0.pool.ntp.org server (use this command only once or as required):
# ntpdate pool.ntp.org

Start the NTP server. The following will continuously adjusts system time from upstream NTP server. No need to run ntpdate:
# /etc/init.d/ntpd start

Configure ntpd (optional)

Edit /etc/ntp.conf, enter:
# vi /etc/ntp.conf
Set public servers from the pool.ntp.org project:

server 0.rhel.pool.ntp.org
server 1.rhel.pool.ntp.org
server 2.rhel.pool.ntp.org

Thailand — th.pool.ntp.org

There are not enough servers in this zone, so we recommend you use the Asia zone (asia.pool.ntp.org):

    server 0.asia.pool.ntp.org
    server 1.asia.pool.ntp.org
    server 2.asia.pool.ntp.org
    server 3.asia.pool.ntp.org