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UT1 NTP Time Dissemination

The Time and Frequency Division of NIST offers a network time service to deliver UT1 time.

The service will be implemented by time servers which transmit UT1 time in standard NTP format. (For example, see RFC 1305 for an explanation of the NTP format.) These servers should respond to time requests in NTP versions 3 or 4 from most standard NTP clients. The servers will treat all requests in client-server mode independent of the actual mode of the request. Broadcast modes are not supported. The reply will have no special indication that the transmitted time is UT1 and not UTC.

The servers will be synchronized to UTC(NIST) with an offset of approximately 1 microsecond RMS. The difference between UTC(NIST) and UTC is typically a few nanoseconds, and is tabulated in Circular T of the BIPM, which is published monthly. ( Rapid UTC (UTCr) gives daily values of the difference between UTC and UTC(NIST) and is computed by the BIPM at more frequent intervals. The servers will compute UT1 time by adding the DUT1 value for any MJD to UTC(NIST). The value used for DUT1 will be the value determined by the IERS ( and published in IERS Schedule A for the MJD of the request when it is received by the servers and with a single linear interpolation between consecutive MJD values.

The accuracy of a time message received by a user will be limited by the stability and reciprocity of the network delay between the user's system and the servers. It will generally be no better than a few milliseconds, but may be much poorer and might be better than this value in some particularly favorable circumstances. This system supports open access and no registration is required.

Here are the server names, locations, and IP addresses for NIST Internet Time Service - the UT1 servers are at the bottom of the page.

Send comments or questions:

Time and Frequency Division NIST Mail Stop 847
325 Broadway Boulder, Colorado 80305
judah.Levine [at] (judah[dot]Levine[at]nist[dot]gov)


Created December 11, 2015, Updated September 13, 2023