http://www.ntp.org/

$ rpm -qa | grep ntp      #查询系统是否安装ntp
$ ntp-4.2.6p5-5.el6.centos.x86_64

$ yum -y install ntp

$ vim /etc/ntp.conf

# restrict default kod nomodify notrap nopeer noquery
restrict default nomodify
# nomodify客户端可以同步
# 将默认时间同步源注释改用可用源
# server 0.centos.pool.ntp.org iburst
# server 1.centos.pool.ntp.org iburst
# server 2.centos.pool.ntp.org iburst
# server 3.centos.pool.ntp.org iburst
server ntp1.aliyun.com    #新加，可是域名 可是ip
server time.nist.gov      #新加，可是域名 可是ip
server 192.168.7.2        #这里是本机地址

$ crontab -e
#*/5 * * * * /usr/sbin/ntpdate time.nist.gov >/dev/null 2>&1

$ ntpq -p

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*ntp1.aliyun.com 10.137.38.86     2 u   22   64    1  525.885  -42.367   0.000
[root@oldboyedu ~]# ntpstat
synchronised to NTP server (110.75.186.247) at stratum 3
   time correct to within 4257 ms
   polling server every 64 s

$ ntpdate 10.0.0.9
 7 Dec 18:43:07 ntpdate[26950]: the NTP socket is in use, exiting

$ ntpdate 192.168.7.2

7 Dec 18:40:16 ntpdate[1453]: step time server 10.0.0.9 offset 40.880807 sec