The following lists contains the time servers we recommend based on their availability.
NTP Associations
An NTP association can be one of the following:
A peer association—The device can either synchronize to another device or allow another device to synchronize to it.
A server association—The device synchronizes to a server.
You need to configure only one end of an association. The other device can automatically establish the association.
All users should ensure that their software NEVER queries a server more frequently than once every 4 seconds. Systems that exceed this rate will be refused service. In extreme cases, systems that exceed this limit may be considered as attempting a denial-of-service attack.
(NTP) is a UDP-based service. NTP servers use well-known port UDP/123 to talk to each other and to NTP clients.
NIST – National Institute of Standards and Technology
| Hostname | IP | Location |
| time-d-g.nist.gov | 129.6.15.27 | NIST, Gaithersburg, Maryland |
| time-a-g.nist.gov | 129.6.15.28 | NIST, Gaithersburg, Maryland |
| time-b-g.nist.gov | 129.6.15.29 | NIST, Gaithersburg, Maryland |
| time-c-g.nist.gov | 129.6.15.30 | NIST, Gaithersburg, Maryland |
| time-a-b.nist.gov | 132.163.96.1 | NIST, Boulder, Colorado |
| time-b-b.nist.gov | 132.163.96.2 | NIST, Boulder, Colorado |
| time-c-b.nist.gov | 132.163.96.3 | NIST, Boulder, Colorado |
| time-d-b.nist.gov | 132.163.96.4 | NIST, Boulder, Colorado |
| time-a-wwv.nist.gov | 132.163.97.1 | WWV, Fort Collins, Colorado |
| time-b-wwv.nist.gov | 132.163.97.2 | WWV, Fort Collins, Colorado |
| time-c-wwv.nist.gov | 132.163.97.3 | WWV, Fort Collins, Colorado |
| time-d-wwv.nist.gov | 132.163.97.4 | WWV, Fort Collins, Colorado |
USNO – United States Naval Observatory
| Hostname | IP | Location |
| tick.usno.navy.mil | 192.5.41.40 | Washington, DC |
| tock.usno.navy.mil | 192.5.41.41 | Washington, DC |
| ntp2.usno.navy.mil | 192.5.41.209 | Washington, DC |
| tick.usnogps.navy.mil | 204.34.198.40 | Colorado Springs, CO |
| tock.usnogps.navy.mil | 204.34.198.41 | Colorado Springs, CO |