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NIST Time Servers

The Time and Frequency Division is an operating unit of the Physics Laboratory of the National Institute of Standards and Technology (NIST). Located in Boulder, Colorado at the NIST Boulder Laboratories, the Time and Frequency Division:

  • Maintains the primary frequency standard for the United States.
  • Develops and operates standards of time and frequency.
  • Coordinates U. S. Time and Frequency standards with other world standards.
  • Provides time and frequency services for United States clientele.

NIST time setting format
The NIST transmits in its own standard Automated Computer Timer Service (ACTS). It is contacted via TCP/IP on port 13. After a setting is made, the time string from the NIST used in the setting is displayed in the NIST log window. ClockWatch translates this string. All times from NIST are in UTC. This time string is made up of a series of fields arranged end to end.

Message Format received from NIST, with an actual sample string below it:

MJD   YYMMDD   HHMMSS   DST LS H ADV  MISC
49010 93-01-23 22:01:22 00  0  0 50.0 UTC(NIST) *

MJD - The first number is the date expressed as a Modified Julian Day number (MJD);  in the above example 49010 is the Modified Julian Day. The Modified Julian Day  is obtained by counting days from the starting point at midnight on 17 November 1858. It is one way of telling what day it is with the least possible ambiguity.

YYMMDD HHMMSS - The next 6 values give the Coordinated Universal date and time (formerly called Greenwich Mean Time) as year, month, day, hour, minute and second.

DST -  The eighth number is the daylight saving time flag, DST.  It is based on the continental US system, which has transitions on the first Sunday in April and the last Sunday in October.
DST = 0 means standard time is currently in effect.
DST = 50 means daylight saving time is currently in effect.
DST = 51 means the transition from standard time to daylight time is at 2am local time today.
DST = 1 means the transition from daylight time to standard time is at 2am local time today.
DST > 51 gives advance notice of the number of days to the transition to daylight time. The DST parameter is decremented at 0000 every day during this advance notice period, and the transition will occur when the parameter reaches 51 as discussed above.
1 < DST < 50 gives advance notice of the number of days to the transition to standard time. The DST parameter is decremented at 0000 every day during this advance notice period, and the transition will occur when the parameter reaches 1 as discussed above. The DST parameter is usually not needed for UNIX systems which keep time internally using Universal Time.
Note: ClockWatch uses the Windows internal Time Zone setting to determine if daylight savings time is both used and in effect.

LS -

H - The health parameter, H, gives the health of the timeserver:
H = 0 means that the server is healthy.
H = 1 means that the server is operating properly but that its time may be in error by up to 5 seconds. This state should change to fully healthy within 10 minutes.
H = 2 means that the server is operating properly but that its time is known to be wrong by more than 5 seconds.
H = 3 means that the hardware or software has failed and that the time error is unknown.

ADV -

MISC - The remaining characters on the line identify the time source and are included for compatibility with the ACTS time system.

*Prepared with information obtained from the NIST, Boulder, CO

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Last reviewed March 19, 2008