root@log-server:~# apt-get install ntp 

root@log-server:~# vim /etc/ntp.conf
[...]
# comentar
#server 0.debian.pool.ntp.org iburst
#server 1.debian.pool.ntp.org iburst
#server 2.debian.pool.ntp.org iburst
#server 3.debian.pool.ntp.org iburst
 
# adicionar
server 0.br.pool.ntp.org iburst
server 1.south-america.pool.ntp.org iburst
server 0.south-america.pool.ntp.org iburst
[...]

root@log-server:~# service ntp restart

root@log-server:~# date
Dom Jul 10 10:49:38 BRT 2016

root@log-server:~# date -s "2016/01/01 00:00"
Sex Jan  1 00:00:00 BRT 2016

root@log-server:~# date
Sex Jan  1 00:00:02 BRT 2016

root@log-server:~# service ntp restart
[ ok ] Stopping NTP server: ntpd.
[ ok ] Starting NTP server: ntpd.

root@log-server:~# date
Dom Jul 10 10:50:26 BRT 2016

root@ns1:~# apt-get install ntp

root@ns1:~# vim /etc/ntp.conf
[...]
server ntp.exemplo.org iburst prefer
[...]

root@ns1:~# service ntp restart
[ ok ] Stopping NTP server: ntpd.
[ ok ] Starting NTP server: ntpd.

root@ns1:~# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 192.0.2.123     .INIT.          16 u    5   64    0    0.000    0.000   0.000