NTP Time servers
0.pool.ntp.org
1.pool.ntp.org
2.pool.ntp.org
3.pool.ntp.org
The NTP Pool DNS system automatically picks time servers which are geographically close for you, but if you want to choose explicitly, there are sub-zones of pool.ntp.org. The “continent†ones are:
Area: HostName:
Worldwide pool.ntp.org
Asia asia.pool.ntp.org
Europe europe.pool.ntp.org
North America north-america.pool.ntp.org
Oceania oceania.pool.ntp.org
South America south america.pool.ntp.org
Also, from NIST time servers:
time.nist.gov
nist1-sj.ustiming.org
nist1-la.ustiming.org
utcnist.colorado.edu
utcnist2.colorado.edu
Reference:
http://support.ntp.org/bin/view/Servers/NTPPoolServers
 http://tf.nist.gov/tf-cgi/servers.cgi