Sync time using NTP in RHEL/CentOS

Install NTP package

#yum install ntp

Update date
# ntpdate pool.ntp.org

Start service 
/etc/init.d/ntpd start
chkconfig ntpd on

Now add command in rc.local file to update date and time in machine reboot time using ntp.

#vim /etc/rc.local   (add in below)

/usr/sbin/ntpdate pool.ntp.org