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Universal Time (UT) Family: Universal Time (UT) is the general designation of time scales based on the rotation of the Earth. In applications in which a precision of a few tenths of a second cannot be tolerated, it is necessary to specify the form of UT such as UT1 which is directly related to polar motion and is proportional to the rotation of the Earth in space. The UT1 is further corrected empirically for annual and semiannual variations in the rotation rate of the earth to obtain UT2.

Universal Time is the mean solar time of the prime meridian plus 12 hours, determined by measuring the angular position of the Earth about its axis. The UT is sometimes designated Greenwich Mean Time (GMT), but this designation should be avoided. Communicators use the designation (Z) or (Zulu). Timekeepers should use UTC of the national standard, for example, UTC(USNO) rather than GMT.

Mean Solar Time is simply apparent solar time corrected for the effects of orbital eccentricity and the tilt of the Earth's axis relative to the ecliptic plane; that is, corrected by the equation of time which is defined as the hour angle of the true Sun minus the hour angle of the mean Sun.

Coordinated Universal Time or Universal Time Coordinated (UTC): A coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM), which forms the basis of a coordinated dissemination of standard frequencies and time signals. NOTE: A UTC clock has the same rate as a Temps Atomique International (TAI) clock or international atomic time clock but differs by an integral number of seconds called leap seconds. The UTC scale is adjusted by the insertion or deletion of seconds (positive or negative leap seconds) to ensure approximate agreement with UT1 (also known as the Julian Date)

Greenwich Mean Time (GMT) is a 24 hour astronomical time system based on the local time at Greenwich, England. GMT can be considered equivalent to Coordinated Universal Time (UTC) when fractions of a second are not important. However, by international agreement, the term UTC is recommended for all general timekeeping applications, and use of the term GMT is discouraged.

Timing Standards

AC Power

The line frequency does vary slightly over the course of the day. (usually decreasing during times of heavy load). At the end of the day, they run slightly faster or slower as needed so that the total line cycles for the day is EXACTLY 5184000. That is, they correct so that the number of line cycles is correct in each midnight-to-midnight period.

Their basic timebase is atomic clocks of some kind (Rubidium oscillators?). There is a network (used to be leased phone lines, maybe satellite, now) of frequency standards which go from regional master clocks, to local grid master clocks, to individual generator's controllers. Most of the slave clocks are Phase-locked-loop or just amplified copies of the master. Seems like they could derive the master oscillator from the GPS signal if they wanted to set a standard, but as far as I know they are independant now.

Why is this done? I mean why does the power line need to be EXACTLY 60Hz?  Most generators are synchronous machines, when they are connected to the grid, you put torque in and they put power into the line. If you load them down, they are motors and draw power from the line. When you start them up, if the rotation speed and phase is not exactly in synch with the grid, they draw huge fault currents until they synchronize or blow the overload breakers open. So they HAVE to be able to synchronize.

Why 60 Hz? I suspect that Charles Steinmetz et al. at Westinghouse picked 60Hz as their standard, based on a tradeoff of transformer losses versus capacitive / resistive losses for their proposed grids based on the technology at the time. Maybe not optimal any more, but there you are. Why 50Hz in Europe?

Why references? If each generator synchronized only to the local grid, there would be phase shifts across the grid and power would flow haphazardly around the grid, causing losses; and there would be no control of who delivered power to whom. The generators control their speed and phase very carefully to put the desired amount of power into the grid. Hence the need for an independent reference oscillator. So, as a side effect of needing accurate synchronization, we have a frequency reference that is widely distributed, and accurate. UNfortunately, the resolution at 60 Hz is poor for many timing tasks. Oh well.

So: For time-of-day clocks, where mains power is available, you _can_ depend on that for long-term accuracy. However, you will run slow when the grid is heavily loaded, and run fast to catch up later. And be dead when the mains fails.

Watch Crystals

Use watch crystals and calibrate them by tweaking the load capacitance when needed to get to +/- 1ppm. ST Microelectronics makes a real-time-clock chip with cool digital frequency correction. (The chip will skip or double pulses to correct for crystal initial tolerance if you calibrate it.) If temperature is not varying wildly, they work very well.

If temperature is unpredictable, the crystal temperature coefficient is a problem. Another reference is needed for very accurate clocks when you cannot predict or control temperature. Another option is to put the crystal in a temperature controlled "oven", if you have the power. See Real Time Methods@ and Clocks@

RF time reference transmitters (WWVB)

As mentioned before, time reference transmitters is one option here in North America, but similar stations may not be available in Europe or Australia, etc. Also:

GPS signals


Internet Accessable Time Standards

NIST Time Servers

A free win 3.1/95/98/NT program that querys the server and corrects the local clock is available from NIST. See

Telnet example: telnet 13

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:

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 by a series of fields arranged end to end.

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

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 Universal Coordinated 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        The next number is the leap second flag, LS.
LS = 0 means no leap second is scheduled.
LS = 1 means that a leap second is to be added as 23:59:60 on the last day of the current month. The last minute will therefore be 61 seconds long. Leap seconds are usually added at the end of either June or December.
LS = 2 means that second 23:59:59 is to be dropped on the last day of the current month. The second following 23:59:58 will be 00:00:00 of the next day. This minute will therefore be 59 seconds long. This situation is unlikely to be necessary in the foreseeable future.
Note that leap seconds are inserted or deleted at the specified Universal Times, while daylight savings transitions are always with respect to local time.

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 have failed and that the time error is unknown.

ADV        The advance parameter, ADV, gives the time advance of the transmissions, in milliseconds. Each time packet is sent out early by this amount to compensate (approximately) for the network delay.

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


The Daytime protocol follows Internet time standard RFC-867. It is contacted via TCP on port 13. The time code is sent as ASCII characters.

Unlike the NIST timeservers, times transmitted from Daytime timeservers are typically sent in the local time where the server is located. The time string is made up by a series of English language time and date fields, Fields are separated with a single space between them.

Typical Message Format received from a Daytime Protocol server, with a sample string below it:

Thu   Aug 27 12:34:28        1998

DoW Day of the week as a 3 letter abbreviation for the English day of the week (i.e. Sun, Mon, Tue, Wed, Thu, Sat)

Mth Month as 3 letter abbreviation of the English month of the year (i.e. Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec)

DD Day of month. Two-digit day of the month.

HH:MM:SS    The next 8 values give the local time hour, minute and second separated with a colon (:)

TZ Optional 3 or 4 letter abbreviation for the local time zone. (i.e. CDST, CEST). ClockWatch does not use this field. Use the difference in hours field on the server option form to correct for a time server in a different time zone.

YYYY The current four-digit year

The dialup time service in the US is known as ACTS (Automated Computer Time Service). Also:

See also:

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