The Story of 24-Hour Watches

Humans are curious creatures by nature. Since the early dawn of our existence, we have questioned every aspect of our place within the universe. Where does the sun go at night and when will it come back? Why does the moon fade away and disappear only to reappear again? Why do the tides rise and then recede?

It is little surprise that we would want to measure the ebb and flow of nature in order to better understand our world. Trying to understand when the rains would fall, when the herds would migrate, how long before the day would break - not only satisfied our curiosity - but was critical in order to ensure our survival.

Perhaps the first great mystery facing early humans was understanding the cycle of our sun, and dividing up our day into segments that could be counted, easily understood and anticipated. Most of us don’t give much thought as to why our day is partitioned into 24 hours, assuming that this is just the way things are, like the sun always rising in the east. But our day has not always been chopped up into these 24 familiar bits throughout all of history. So where did the idea originate?

Our world has seen a variety of timekeeping formats throughout recorded history, spanning a numerous cultures and time periods. In ancient China, during the Han Dynasty, daytime was subdivided into 15 hours called shí. Each of these periods was named for an activity or event that would correspond with it, such as “dawn” (chénmíng), “daybreak” (danming), “early meal” (zaoshi), “noon” (zhèngzhōng), “short shadow” (shaohuan), “evening” (būshí), “long shadow” (dahuan), “sunset” (xiandong), and “rest time” (dinghun). Evening hours were marked in periods called gēngs, meaning “watch,” named after the watchmen who would stand guard over a town and mark each passing hour with a gong.

Once the day and night had been effectively divided, the next logical step would be the invention of devices to track and time its passing. The earliest timekeeper in most societies is the sundial, which can track the passing of time during clear daylight hours. In order to track the hours after sunset, various timekeeping devices were created to measure the hours. The water clock, hourglass, candle and incense clock were all used in various places, in order to count the hours until sunrise. As the length of daylight varies throughout the year, so too did the length of the hours that measured daytime and nighttime. During the summer months, the hours of the day would grow much longer while hours at night would shorten. This process would naturally reverse itself during the winter months. The devices used to measure the passage of time would need to be calibrated throughout the year to reflect these changes.

The idea to use 24 hours to measure a day can be attributed to the ancient Egyptians. It was millenia ago along the banks of the Nile, that pioneering astronomers would divide the daylight into ten partitions measured by sundials. These ten daytime hours would receive two additional hours for the morning and evening twilights, bringing a total of 12 hours of light. Conversely, nighttime would also be divided into 12 hours, bringing the total to 24 hours for one full day. Since the hours could not be measured by a sundial at night, the ancient Egyptians devised a way to mark the passing time until sunrise by observing the overhead rotation of the stars. Star groups called “decans” were observed and their rising and setting over the horizon could determine the length of time until sunrise.

Although the Egyptians are primarily responsible for our 24-hour day, we have the ancient Babylonians to thank for our 60-second minute. This is due to the fact that the Babylonians used a numbering system which used 60 as a base, rather than 100 as we are familiar with. This base-60 system also explains why we still have 360 degrees in a circle.

Despite the fact that the 24-hour day became the standard over the centuries, it doesn’t mean that there haven’t been attempts to reform and improve upon the format. During the French Revolution, many reformists were captivated with the idea of reinventing and restructuring society from the highest levels, down to the basic foundations of everyday life. Once in power, the revolutionaries altered the calendar by changing the names of the months and days to purge them of any religious or royal significance. In addition, the week was expanded to ten days in order for the year to be set on a more logical, base-100 metric system. This aggressive reform - aimed directly at sweeping away the last vestiges of the old regime - also expanded to include the 24-hour day.

As a result of these reform measures, the new government in Paris introduced decimal time, which is a system based on a ten-hour day divided into 100 minutes per hour and 1000 seconds per minute. Although this new method would be abandoned in 1805, there are a few clocks that still remain from the brief period when decimal time was the official timekeeping system of France.

A French clock featuring a decimal and a 24-hour dial. Circa 1800, made by Pierre Daniel Destigny.


Also interested in measuring time in a new way, the Swiss conglomerate, Swatch, has recently resurrected decimal time, by introducing what is called “.beat time” in 1998. The system involves dividing a day into what are called .beats, which are equal to the French Revolutionary minute, or 86.4 seconds in standard time. Also like the French Revolutionary system, there are 1000 .beats per day. One of the advantages of .beat time is that it does not utilize time zones, making international communications much more efficient.

A Swatch digital watch displaying both standard time at the top and beat time below.

Although the 24-hour day eventually became standard - warding off threats from zealous French revolutionaries and Swiss time moguls - the debate on how to display and refer to those 24 hours was far from settled. When mechanical clocks began to appear throughout Europe in the 1300s, in certain regions it was prefered that the dial reflect the two halves of the day. To accomplish this, some clocks featured dials with two 12-hour scales in sequence, and an hour hand making one rotation a day. This type of display is called the “double XII” system for its use of two different “XII” markers beginning each 12-hour scale.

The famous Exeter Cathedral Clock in Devon, England, featuring a double XII dial.

Over time, the redundancy of two identical 12-hour tracks in succession would become apparent, and the dial evolved to a display featuring only one 12-hour scale with an hour hand that made two revolutions around the dial a day. To differentiate between the different hours in speech, the Latin terms ante meridiem (a.m.) and post meridiem (p.m.) would be employed. Meaning “before midday” and “after midday” respectively, the a.m./p.m. system would become the preferred system primarily in the English speaking world.

As the a.m./p.m. system gained popularity in certain parts of the world, some flaws regarding its use became apparent. Since 12:00 serves as both the beginning and end point for both noon and midnight, confusion between the two became common. At various places and at different times throughout history, 12:00 p.m. and 12:00 a.m. have both been used to refer to midnight. In colloquial speech, differentiators such as, “12 midnight” or “12 noon” would need to be added. While this is an acceptable solution for oral communication, confusion still remains in cases dealing with printed information, In these instances, 11:59 p.m. or 12:01 a.m. will often be used in timetables or schedules to eliminate potential confusion.

Throughout the rest of the world, the system known as 24-hour time, became the preferred timekeeping method. Known as “military time” in the English-speaking world, 24-hour time eliminates all confusion by getting rid of the 12-hour a.m./p.m. distinction and simply numbering all hours from 0 to 24. Midnight, for example is referred to as 00:00, 6:00 a.m. is written 06:00, noon is 12:00, but instead of returning to 1:00 for 1:00 p.m., 24-hour time continues on to 13:00. 6:00 p.m. is 18:00 and the day ends at midnight or 24:00 which is the same as 00:00.

The advantages inherent to this system - including its ability to reduce confusion - makes it the perfect choice for a number of professions, even within the English-speaking world. Pilots, military personnel, and frequent travelers who often cross timezones and need to keep track of their home time, would benefit greatly from 24-hour timekeepers. Additionally scientist and those in the medical profession who must avoid ambiguity, frequently make use of 24-hour clocks and watches.

While much of the world has adopted the 24-hour time system, the vast majority of analog displays still utilize the 12-hour dial. Some timepieces, however, include a second chapter of numerals from 13 to 24, usually below 1 to 12. Watches with this type of display are frequent called “military dials” and are produced by a number of manufacturers. But a true 24-hour analog dial, is something of a unique find in today’s world of watch design. Perhaps it is the fact that they are so rare, and present the wearer with an entirely different way to view time, that 24-hour watches have become a sought-after addition for many collectors.

There are a few well-known examples of 24-hour dials in various parts of the world. One of the most famous is the Shepherd Gate Clock, which can be found hanging on the wall outside the Royal Greenwich Observatory. This clock is most likely the first timepiece to ever display Greenwich Mean Time to the public. During World War II, the dial was damaged during a German bombing raid, but the historic clockwork survived and the dial was replaced with an exact duplicate.

The Shepherd Gate Clock at Greenwich.

One of the reasons why 24-hour dials are so rarely encountered on a wristwatch has to do with mechanics and economics. Since the overwhelmingly vast majority of timepieces utilize 12-hour dials, almost every horological manufacturer produces only movements suitable for this typical watch. Converting a watch movement to run on a 24-hour dial includes additional steps, and requires specialized tooling that add both time and cost to the ordinary production process. In order to better understand how a movement is transformed for the hour hand to rotate around the dial once a day instead of twice, it would be helpful to take a closer look at the mechanism itself.

Here, the dial side of a Unitas 6325 can be seen to illustrate some of the components involved with moving the hands. At the center are the parts known as the cannon pinion, hour wheel and minute wheel. All three of these must work together to control how the hands move around the dial.

Reconfiguring a movement to function as a 24-hour timepiece requires costly retooling of the machinery that fabricates these parts. By cutting a different amount of teeth and in a different size, the gear ratios are altered, allowing the hour hand to rotate only once around the dial in a day, while the minute hand remains the same at one rotation per hour.

It is unlikely that the average person will encounter very many 24-hour watches in their day to day life. But for the adventurous spirit, few watches can claim to be as unique as the people who wear it. That’s just one of the many things that makes Akerfalk the perfect timepiece for those who view time - and life - just a little bit different.


Photo Credits

Decimal Clock - DeFacto CC BY-SA 4.0
Digital Swatch with .beat time - Public Domain
Exeter Cathedral Clock - DeFacto CC BY-SA 4.0
Shepherd Gate Clock - Alvesgaspar CC BY-SA 3.0
Unitas Movement - Eric Gregoire
Akerfalk Watch - Akerfalk AB

Special thanks to Eric Gregoire