How precise is my timepiece?

Ever had this experience: You show up for an important meeting, and your watch tells you that you're exactly on time. Yet, to your surprise, the receptionist comments that they've been wondering if you'd make it. She points to the wall clock, which shows you're five minutes behind.

After getting over the initial embarrassment, you likely ask yourself: How did this happen? You're the type of person who buys a watch and immediately calls the U.S. Naval Observatory's "correct time" number (202-762-1401) to set it to the exact second. So, how does your watch end up being slightly off? And sometimes, it's not just a little – or so your friends claim. This raises another question: Whose watch is right – yours or theirs? And more importantly, how accurate can any watch be? It makes you want to throw your hands up and belt out Chicago's 1969 hit "Does anybody really know what time it is?" (Not in the mood for a sing-along?)

Well, take solace in knowing you're not the first person to be puzzled by personal timekeeping. It’s a confusing subject. But we're going to get to the bottom of it. Let's begin by exploring the history of the watch.

A Short History of (Accurate) Timekeeping

As you might imagine, early humans didn't have watches. They didn't really need them either, as their nomadic hunter-gatherer lifestyle didn't involve catching trains or tracking hourly wages. However, as civilizations advanced and labor became more specialized, the need for synchronization grew. Sundials, which measured the sun’s shadow, were one of the first timekeeping inventions. The Egyptians, focused on nighttime rituals, created the water clock -- essentially a large vase with a hole in the bottom that measured time in hourly drips [source: Woods]. In Medieval Europe, mechanical clocks appeared in the 1300s, making accurate timekeeping possible. Early mechanical clocks could only measure time to within 15 minutes, but in the late 1600s, Dutch scientist Christiaan Huygens invented the pendulum clock, which was accurate to within 10 seconds per day [sources: Lienhard, NIST]. By the 1850s, American Watch Co. in Waltham, Mass., introduced mass-produced spring-powered pocket watches, which allowed people to carry time with them wherever they went [source: NMAH].

But once people had clocks and watches, there was a new problem: What time should everyone follow? In 19th-century America, each town had its own local time, based on the noon of its own solar clock. This meant that when it was noon in Chicago, it was 11:40 a.m. in St. Louis and 12:18 in Detroit. This created confusion, especially for the rapidly expanding railroad industry, which needed reliable, standardized schedules [source: Mansfield]. The railroads used time set by celestial observations from the Harvard College Observatory, transmitted via telegraph [source: NMAH]. In 1883, to standardize time, railroad companies divided the U.S. into four time zones and required cities to adjust their clocks accordingly, or face economic isolation. People in Maine were frustrated with the new "Philadelphia Time," but eventually, the whole country became synchronized [source: Mansfield].

In the 20th century, scientists developed ultra-precise clocks based on the vibrations of crystals and even individual atoms. These innovations allowed time to be measured in fractions so small — down to the trillionths of a second — that they are beyond our normal perception [source: NMAH]. That’s why we all have the same time on our watches, and we’re all supposed to be punctual for our appointments today. Except, of course, we’re not. So what’s going on?

How much does the accuracy of a watch vary, and what causes this variation?

In theory, we should all be perfectly synchronized. With the introduction of battery-powered quartz wristwatches in the early 1970s, ordinary people gained access to a timekeeping technology once reserved for scientists and technicians [source: NMAH]. Essentially, applying electricity to a small piece of quartz and then bending it causes the crystal to emit a stable electrical signal, which can drive an electronic clock face [source: NIST]. By the early 2000s, quartz watches were so widespread that mechanical watches made up only 13 percent of the global market for timepieces [source: IEEE].

However, consumer-grade quartz watches aren't perfectly precise. Remember, these are relatively inexpensive devices, mass-produced in large quantities rather than being custom-made for laboratory settings. Even the most costly quartz watches still depend on a mechanical vibration whose frequency can vary depending on factors like the size and shape of the crystal. No two quartz crystals are identical, which means slight differences can arise between watches from the same production line [source: NIST]. Additionally, things like temperature, humidity, and wear-and-tear can affect a watch's precision by influencing the stability of the tiny motors that generate the electric field for the crystals [source: Lombardi].

As a result, quartz watches gradually lose some of their accuracy over time -- though it’s important to note that this loss is usually minimal. According to Chronocentric.com, a site dedicated to watch enthusiasts, consumer quartz watches typically lose between a tenth of a second and two seconds daily. While this discrepancy might not seem significant, if left unchecked over a long period, it could cause a watch to fall behind by a few minutes [source: Chronocentric.com]. A 2008 study in the Horological Journal, however, found that some inexpensive watches were far more accurate. The researchers tested everything from a counterfeit Rolex bought for $15 to a $30 Timex from a discount store, and all of them were accurate to within a few thousandths of a second each day. Over the course of years, such a tiny difference would only be noticeable to their owners after much longer periods of time [source: Lombardi].

How to Set Your Watch for Precision

If even budget quartz watches are accurate within a fraction of a second each day, then why was your watch so far off when you arrived for your appointment? The most likely explanation is that you either didn’t set it correctly from the start, or you’ve been wearing it for a long time, subjecting it to temperature swings and humidity that affected its performance. But there's no need to buy a new watch. A simple fix would be to check it periodically against a reliable reference and reset it when needed.

If you're in the U.S., you can verify the correct time by contacting the National Institute of Standards and Technology, which operates two radio stations — one in Colorado and the other in Hawaii — that broadcast a continuous time signal. The Colorado station can be reached at (303) 449-7111, while the Hawaii station can be dialed at (808) 335-4363. The time signal is accurate to within 30 milliseconds, which accounts for the delay due to the telephone connection across the country [source: NIST].

The official U.S. time, based on NIST and the U.S. Naval Observatory in Washington, D.C., can also be accessed online at www.time.gov [source: NIST]. NIST also offers a free program that synchronizes your Windows computer with the official U.S. clock [source: NIST].

Outside the U.S., you can set your watch by visiting www.worldtimeserver.com, a site sponsored by a software company that receives its time data from the Royal Observatory at Greenwich in the UK.

By regularly checking with these time sources, you might just find that you’re never late for an important appointment again.