History of the 24-Hour Clock

History of the 24-Hour Clock

Where it all began

It is believed that the Egyptians are the origin of the 24-hour system of time. They used what is known as the Egyptian astronomical decan system. Decans are small constellations totaling 36 groups of stars. These decans would rise on the horizon during each earth rotation. This rising would mark the start of a new “hour” during the night time. This Egyptian time system of decans would continue to be used by navigators, astronomers, and scientists for centuries.

Italy was the first to introduce the world to the mechanical 24-hour public clock. This clock counted each of the 24 hours in the day. The clocks were designed to produce a total of 300 strokes per day which meant they used a lot of rope and would wear down quickly. Their solution was to reduce the amount of strokes each day from 300 strokes to 156 strokes and eventually to 84 strokes per day.

The worldwide adoption of the 24-hour clock

In 1886, the Canadian Pacific Railway train at Port Arthur began using the 24-hour clock. However, in 1893 Italy became the first to adopt the 24-hour clock nationally. The French Army began using the 24-hour clock in 1909 whereas the rest of France didn’t start using this time system until 1912. The British Royal Navy began using this time system in 1915 during the First World War and the Allied forces would follow suit as well. Each country began to follow suit and adopt the 24-hour clock time system as their national clock. Denmark adopted it in 1916 with Greece adopting this system the very next year (1917). However, the Canadian armed forces would not choose it until late 1917. In 1918; the British Army adopted the 24-hour clock. The United States Navy was the very first organization in the U.S.A. to utilize the 24-hour clock, and they did so in 1920. Spain, Belgium, Portugal, and Switzerland made the switch by the year 1920. In fact, most of the countries in Latin America had switched to the 24-hour time system in the early 1920’s. Turkey followed in 1925 and Germany would change in 1927.  While the United States Navy had already adopted the 24-hour clock in 1920, the United States Army would not begin using it until July 1, 1942, near the start of World War II.

What is the 24-Hour Clock?

The 24-hour clock is a way of marking time from midnight to midnight. The clock marks each hour starting at midnight (00:00 or 24:00) to 11 p.m. (23:00). However, midnight can also be marked 24:00 as it is not only the last hour of a day but, it is also the first hour of the following day. The 24-hour clock time can be written in two ways. One of them being hh:mm this represents the exact hour of the day as well as the exact minute. For example, it could be 24:45 (translated as 12:45 a.m.). Alternatively, the 24-hour clock time can also be written as hh:mm:ss which represents the exact hour of the day, the exact minute of the day as well as the exact second of the day. For example, you might see 22:36:14 (translated as 10:36:14 p.m.).

Some parts of the world may log times after 24:00 (midnight). For example, you could travel to Japan and watch broadcast television at 1:30 a.m. and the time read 25:30 instead of 01:30. This is due to an unusual context where certain business hours may range beyond midnight (as broadcast television does). Places, where you might encounter this rare way of telling time, include Japan, the United Kingdom, Hong Kong, South Korea, and China.

Why is the 24-Hour Clock referred to as military time?

The 24-hour clock seems to be referred to as “military time” in English speaking parts of the United States and Canada. The reason being that we strictly follow the 12-hour clock system rather than the 24-hour clock system. The exception to the rule being our armed forces and other authoritative areas of the country (aviation, navigation, meteorology, emergency services and astronomy).

The general population, however, follows the 12-hour clock system which marks midnight as 12:00 a.m. and is followed by 1:00 a.m., etc. until noon which is marked 12:00 p.m. and is followed by 1:00 p.m., etc. until midnight is reached again. The a.m. and p.m. separate the two repetitions of 12 to 11:59.

For the United States military as well as the various other English-speaking armed forces, there are specific changes that are made to the 24-hour clock system that other areas of the world may not recognize. For example, they believe that there should always be leading zeros in written and in spoken time. Therefore, the time 3:45 a.m. would not be written or spoken as such. It would be spoken zero three forty-five if it were a casual conversation or it would be spoken as zero three four five if it were being spoken over military radio. The way this time would be written is different as it not only adds the zero but, it also removes the hour/minute separator and the time zone’s designating letter would be added as well. For example, the time 3:45 a.m. would be written 0345Z. This letter is added because all Military time zones are lettered. The letter then corresponds with the military alphabet. For example, if it were 3:45 a.m. Greenwich Mean Time or Coordinated Universal Time then it would be written 0345Z (“Z” is the letter for the time zone and corresponds with the code word “Zulu”, thus making it  “zero three four five Zulu time”. The last change that is made for our military time is the way the exact hours are spoken. They are not referred to as “thousands” but rather as “hundreds”. 24:00 (2400) is expressed “twenty-four hundred” instead of “two thousand four hundred”. The same applies to the other times such as 10:00 (1000) is expressed “ten hundred” instead of “one thousand” and so on.


The 24-hour clock may have been one of the original methods of telling time, but today it is simply used by authoritative areas such as the armed forces. Today, it is more common to see the time displayed in the 12-hour clock system as there is little reason for us to follow the original 24-hour clock.