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Time References for Setting
Watches and Clocks

1998,2006 by Marc J. Rochkind. All Rights Reserved. Information here may be freely used for personal, non-commercial purposes.

To set your watch or clock accurately, it's convenient to use a time reference that is:

  • available whenever you want it,
  • accurate, and
  • precise.

This page explains the various options for residents of North America. If we learn of links to information about time references in Europe, Asia, and other parts of the world, we'll certainly include them. Some of the information here, such as that pertaining to short-wave, does apply globally.

Principal US Time References

There are two principal US government agencies that provide a time reference based on atomic clocks:

USNO keeps the official time, but NIST provides more ways for the public to access the time. For all practical purposes, the times are the same.

There are ways to keep time accurately without relying on NIST or USNO, such as owning your own atomic clock, but for our purposes these two agencies will suffice.

Accessing the Time

You can access the time directly, which means that you are listening to a NIST or USNO time signal, or indirectly, which means that you are using a clock that has been set automatically from the NIST or USNO time signals.

Without considering exotic laboratory equipment, the practical ways to access the time signal directly are via radio, telephone, or a Web page. The indirect way is to use a clock that has been set from the time signal. Each of the ways is discussed separately below.

Audible Radio (Direct)

NIST has two time-signal radio stations: WWV in Colorado, and WWVH in Hawaii. Both broadcast continuously on 2.5, 5, 10, and 15 MHz. WWV also broadcasts at 20 MHz. Any short-wave radio can receive these broadcasts. There is a voice announcement of the time, and a tone that marks the exact minute. (There are other signals as well, but you don't need them for setting your watch or clock.) While there are various factors that cause errors in reception, the tone you hear is accurate to within a millisecond.

There's loads of information about NIST's time services in NIST Special Publication 432.

Audible Telephone (Direct)

Both NIST and USNO operate telephone services that you can dial to hear the time broadcast. The error is negligible for land-based transmission, but could be as much as a half-second if a satellite is used. These are the numbers:

Number Source Comments
202-762-1401 USNO land or satellite; no charge other than cost of call
202-762-1069 USNO land or satellite; no charge other than cost of call
900-410-8463 USNO land guaranteed; 50 first minute; 45 additional minutes
303-499-7111 NIST land or satellite; no charge other than cost of call

Web Clock (Direct)

NIST has a Java-coded clock here. The error caused by network delays can be around a second. Also, check out Atomic Clock Time Cover Page, which has links to several Web clocks.

Radio-Controlled Clock (Indirect)

For as little as $50 you can buy a quartz clock that sets itself automatically from WWVB, a low-frequency NIST broadcast originating from Colorado at 60 KHz . Some clocks set themselves once an hour, some once every four hours, and some once a day. Regardless of the interval, the clock should be less than a second off at any time, although clocks that set themselves more often are better. The clock part isn't special, so these are not "the world's most accurate clocks," despite the way they're advertised. Rather, they are just set more often and more accurately than conventional clocks. Unlike the audible NIST and USNO time signals, a radio-controlled clock is useful in its own right, in addition to being a time reference for setting another watch or clock.

Here are some sources of radio-controlled clocks (there are also Junghans and Arcron Zeit radio-controlled watches, but these aren't listed). (2006 Update: The choices and prices are way off for 2006. I bought a radio-controlled clock at a Walgreens a few weeks ago for $10, and it wasn't even on sale.)

Clock Approx. Price Source
Arcron Zeit Wall Clock $100 - $130 Order from zeit
Arcron Zeit Executive (Desk) Clock $130 Same as above
Arcron Zeit Travel Clock $90 Same as above
Junghans MEGA Desk Clock $250 Jewelry stores;
specialty stores like Sharper Image and Brookstone;
mail order catalogs
Junghans MEGA Alarm Clock $100 Same as above
Oregon Scientific Radio Controlled Clock $100 Information at Oregon Scientific
Oregon Scientific Radio Controlled Projection Clock $80 Same as above
Radio Shack Super-Accurate Auto-Set Digital Clock $50 Any Radio Shack store

Computer Clock (Indirect)

There are several utilities that set your computer clock to the NIST time reference via the Internet. These are terrific for setting your computer clock, but not so good for setting watches and other clocks, because computer clocks are very inaccurate. (2006 Update: This feature seems to be built into OSes these days--you probably don't need a utility.)

Global Positioning System

The Global Positioning System (GPS) consists of 24 satellites  that provide position, velocity, and time data to receivers that cost as little as $100 (for the Magellan GPS Pioneer). Other consumer receivers are made by Garmin, Trimble, and others. GPS receivers are very easy to buy--I found several Magellan models at my local K-Mart. (2006 Update: I'm sure the prices have dropped and the availability improved.)

The GPS satellites don't just rebroadcast a time signal coming from somewhere else. Each satellite contains its own set of atomic clocks. While the time signal sent is very accurate, and GPS receivers process it accurately when they compute position, the time as displayed may be off by a second or two. At any rate, that is the experience reported by some owners of low-priced receivers, as well as my own experience with the Magellan Pioneer. A plausible reason for the inaccuracy is that the internal clock is set only after the positional calculations are completed, which are the first priority, and by then some time has elapsed. I don't know whether more expensive receivers display the time more accurately--it is, in principle, possible for them to do so.

To access the time, you just turn on your GPS receiver, give it a minute or two to find some satellites (my receiver needs at least three), and then look at the time display. On the Magellan Pioneer you press the Menu key a few times to get to the time display; other receivers probably operate similarly.

Additional Links

Both NIST and USNO maintain lists of links to other time-related sites:

NIST Links (no longer works, but main NIST link at top of page does)

USNO Links (works as of 27-March-2006)

There's lots of of GPS information on the web as well:

Aerospace Corporation GPS Primer (no longer works)

Coast Guard GPS Information (no longer works)


(40253 hits when counter was turned off on 2-June-2008)

Last Updated: 27-Mar-2006 04:01:10 PM (minor update to fix links and a few other things; last content update was 19-July-2003)

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