CentOS:
0.centos.pool.ntp.org
1.centos.pool.ntp.org
2.centos.pool.ntp.org

国内可用的

ntp.fudan.edu.cn 复旦
210.72.145.44 国家守势中心服务器

亚洲地区的ntp服务器

Bangladesh — bd.pool.ntp.org (0)
China — cn.pool.ntp.org (3)
Hong Kong — hk.pool.ntp.org (1)
India — in.pool.ntp.org (2)
Indonesia — id.pool.ntp.org (2)
Iran — ir.pool.ntp.org (0)
Israel — il.pool.ntp.org (3)
Japan — jp.pool.ntp.org (5)
Korea — kr.pool.ntp.org (4)
Malaysia — my.pool.ntp.org (3)
Philippines — ph.pool.ntp.org (0)
Singapore — sg.pool.ntp.org (3)
Taiwan — tw.pool.ntp.org (7)
Thailand — th.pool.ntp.org (1)
Turkey — tr.pool.ntp.org (1)
United Arab Emirates — ae.pool.ntp.org (0)

国外的:

time.nist.gov  美国
time-a.nist.gov
time-b.nist.gov
time-nw.nist.gov