1.在不装ntp时，ntpdate 命令是可以用的，且能与ntp服务器同步时间
root@wilson-software:~# date -s "18:22"
Tue Jul 5 18:22:00 CST 2016
root@wilson-software:~# ntpdate -d 202.108.6.95
5 Jul 18:22:12 ntpdate[4439]: ntpdate 4.2.6p3@1.2290-o Tue Jun 5 20:12:12 UTC 2012 (1)
Looking for host 202.108.6.95 and service ntp
host found : xk-6-95-a8.bta.net.cn
transmit(202.108.6.95)
receive(202.108.6.95)
transmit(202.108.6.95)
receive(202.108.6.95)
transmit(202.108.6.95)
receive(202.108.6.95)
transmit(202.108.6.95)
receive(202.108.6.95)
transmit(202.108.6.95)
server 202.108.6.95, port 123
stratum 2, precision -23, leap 00, trust 000
refid [202.108.6.95], delay 0.18800, dispersion 0.04358
transmitted 4, in filter 4
reference time: db26271d.a0ac71ba Tue, Jul 5 2016 20:31:25.627
originate timestamp: db262ab0.7833e87e Tue, Jul 5 2016 20:46:40.469
transmit timestamp: db2608db.c66385a5 Tue, Jul 5 2016 18:22:19.774
filter delay: 0.32901 0.32188 0.25488 0.18800
0.00000 0.00000 0.00000 0.00000
filter offset: 8660.688 8660.677 8660.649 8660.613
0.000000 0.000000 0.000000 0.000000
delay 0.18800, dispersion 0.04358
offset 8660.613347
5 Jul 18:22:21 ntpdate[4439]: step time server 202.108.6.95 offset 8660.613347 sec
root@wilson-software:~# date
Tue Jul 5 18:22:30 CST 2016
root@wilson-software:~#
如上所示，设置错误时间后，用ntpdate -d 202.108.6.95 只能调试模式测试能否同步时间，但不会真正同步时间
root@wilson-software:~# date
Tue Jul 5 18:22:30 CST 2016
root@wilson-software:~# ntpdate 202.108.6.95
5 Jul 20:52:33 ntpdate[4465]: step time server 202.108.6.95 offset 8660.555090 sec
root@wilson-software:~# date
Tue Jul 5 20:52:49 CST 2016
ntpdate 202.108.6.95 可以同步时间
2.安装ntp后，ntp服务已经起来的情况下ntpdate 命令是不能用的，即不能通过ntpdate命令与其他时间服务器同步
root@wilson-software:~# date
Tue Jul 5 20:57:07 CST 2016
root@wilson-software:~# date -s "18:18"
Tue Jul 5 18:18:00 CST 2016
root@wilson-software:~# ntpdate 202.108.6.95
5 Jul 18:18:09 ntpdate[5081]: the NTP socket is in use, exiting
root@wilson-software:~# ps -ef|grep ntp
ntp 5067 1 0 18:17 ? 00:00:00 /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u 120:129
root 5091 2877 0 18:20 pts/0 00:00:00 grep --color=auto ntp
root@wilson-software:~# date
Tue Jul 5 18:20:31 CST 2016
root@wilson-software:~#
3.机器启动时ntp服务会起动并会与/etc/ntp.conf里配置的server同步时间（网上有人说时间比服务器快，则不能同步，证实是错误的）
4.具体参考：http://www.linuxdiyf.com/linux/10534.html
5.这行是时间服务器的层次。设为0则为顶级，执行ntpq -p是无效的，如果要向别的NTP服务器更新时间，请不要把它设为0
fudge 127.127.1.0 stratum 0 stratum
root@wilson-software:~# ntpq -p
localhost: timed out, nothing received
***Request timed out
root@wilson-software:~# vi /etc/ntp.conf
root@wilson-software:~# /etc/init.d/ntp restart
* Stopping NTP server ntpd [ OK ]
* Starting NTP server ntpd [ OK ]
root@wilson-software:~# ntpq -p
remote refid st t when poll reach delay offset jitter
time7.aliyun.co .INIT. 16 u - 64 0 0.000 0.000 0.001
dns1.synet.edu. .INIT. 16 u - 64 0 0.000 0.000 0.001
news.neu.edu.cn .INIT. 16 u - 64 0 0.000 0.000 0.001
time6.aliyun.co .INIT. 16 u - 64 0 0.000 0.000 0.001
juniperberry.ca .INIT. 16 u - 64 0 0.000 0.000 0.001