1. Instale o pacote ntp com o utilitário YUM (CentOS / Red Hat) ou o APT (Debian):

# yum install ntp -y
Ou:
# apt-get install ntp -y

2. Em seguida, liste os comandos de administração fornecidos pelo pacote ntp:

# ntp [tecle duas vezes Tab]

  ntpd   ntpdate   ntpdc   ntp-keygen   ntpq   ntpstat   ntptime
  
   

3. Inicialize o serviço:

# service ntpd start
Ou:
# /etc/rc.d/init.d/ntpd start

Ou, ainda:

# /etc/init.d/ntp start

4. Ao iniciar o serviço, analise em seguida, os logs no /var/log/messages:

# tail /var/log/messages
Ou:
# grep ntpd /var/log/messages

 Jul 23 13:11:51 labcentos01 yum[9343]: 
  Installed: ntpdate-4.2.4p8-3.el6.centos.x86_64
 Jul 23 13:12:11 labcentos01 ntpd[9375]: 
  ntpd 4.2.4p8@1.1612-o Fri Feb 22 11:23:27 UTC 2013 (1)
 Jul 23 13:12:11 labcentos01 ntpd[9376]: 
  precision = 0.111 usec
 Jul 23 13:12:11 labcentos01 ntpd[9376]: 
  Listening on interface #0 wildcard, 0.0.0.0#123 Disabled
 Jul 23 13:12:11 labcentos01 ntpd[9376]: 
  Listening on interface #1 wildcard, ::#123 Disabled
 Jul 23 13:12:11 labcentos01 ntpd[9376]: 
  Listening on interface #2 lo, ::1#123 Enabled
 Jul 23 13:12:11 labcentos01 ntpd[9376]:   
  Listening on interface #3 eth0, fe80::a00:27ff:fe6b:d113#123 Enabled
 Jul 23 13:12:11 labcentos01 ntpd[9376]:   
  Listening on interface #4 lo, 127.0.0.1#123 Enabled
   
   

5. Automatize o serviço para que inicie no boot:

# chkconfig --level 35 ntpd on
Ou:
# update-rc.d ntp defaults