Quick config snippet to setup external NTP server to synchronize time on Cisco IOS router.

labisr-01(config)#ip name-server
labisr-01(config)#ntp server 0.europe.pool.ntp.org
Translating "0.europe.pool.ntp.org"...domain server ( [OK]
labisr-01(config)#clock timezone GMT 0
Please note that first I setup nameserver on router to be able to resolve NTP server FQDN.

Once NTP server is set we can check status of it:

labisr-01#sh ntp associations
address         ref clock     st  when  poll reach  delay  offset    disp
*~    3    37    64  377    38.7    2.04     1.4
* master (synced), # master (unsynced), + selected, - candidate, ~ configured
labisr-01#sh ntp status
Clock is synchronized, stratum 4, reference is
nominal freq is 250.0000 Hz, actual freq is 250.0000 Hz, precision is 2**18
reference time is D442C30E.7A93ED17 (22:58:22.478 GMT Mon Nov 5 2012)
clock offset is 2.0449 msec, root delay is 71.69 msec
root dispersion is 61.25 msec, peer dispersion is 1.36 msec