24-hour clock facts for kids

The modern 24-hour clock is a way to tell time where the day is divided into 24 hours. It starts at midnight (00:00) and continues until the end of the day (23:59). Sometimes, 24:00 is used to show the exact end of a day. This system is the most common way to write time around the world today. It is also used by the international standard ISO 8601, which helps everyone understand time the same way.

Some countries, especially those where English is spoken, use the 12-hour clock (like 1 p.m. or 2 a.m.). However, even in these places, certain jobs often use the 24-hour clock. For example, in medicine, doctors and nurses use the 24-hour clock to record when things happen. This helps avoid any confusion about whether an event was in the morning or evening.

Understanding the 24-Hour Clock

This map shows how different countries use the 12-hour or 24-hour clock.

A special clock from World War II that shows time in 24-hour format.

A digital clock in Taiwan showing the time using the 24-hour system.

A public clock in Brazil that shows all 24 hours on its face.

A time of day is usually written in the 24-hour system as hh:mm (like 01:23) or hh:mm:ss (like 01:23:45). Here, "hh" means the hours (from 00 to 23) since midnight. "mm" means the minutes (from 00 to 59) since the last full hour. "ss" means the seconds (from 00 to 59) since the last full minute.

To show the exact end of a day, "hh" can be 24, with "mm" and "ss" both being 00. For numbers under 10, a zero is often added in front (like 01 instead of 1). This is very common in computer programs and when specific rules require it.

If you need to be even more precise, seconds can have decimal parts, like 01:23:45.678. The colon (:) is the most common symbol used to separate hours, minutes, and seconds. In some special cases, like certain computer systems or military communications, no separator is used (for example, "2359").

Midnight: 00:00 vs. 24:00

In the 24-hour system, the day starts at midnight, which is 00:00. The last minute of the day begins at 23:59. The notation 24:00 can also be used to mean midnight at the very end of a specific day. This means 24:00 on one day is the same as 00:00 on the next day.

Using 24:00 is helpful when talking about time intervals. For example, a store might say its hours are "07:00–24:00," meaning it's open until the very end of the day. Bus and train schedules also use 00:00 for departure and 24:00 for arrival to be very clear.

Time Beyond Midnight

Sometimes, for special schedules like TV broadcasts or public transport that run past midnight, you might see times like 25:00 or 26:00. These are not standard ways to tell time. They are used to show that an event continues into the next day without resetting the clock to 00:00.

Computers and 24-Hour Time

Most computers around the world show time in the 24-hour format by default. For example, if you set up a new computer, it will likely use the 24-hour clock unless you choose a specific language or region that prefers the 12-hour system. This is because the 24-hour system is very clear and avoids confusion.

Military Time: A Special Use

In the United States, where the 12-hour clock is common, the 24-hour clock is often called military time. It's used in fields like aviation, navigation, and emergency services. This is because being super clear about time is very important in these jobs.

Military forces, especially those working together from different countries, have special rules for using the 24-hour clock:

• They don't use a colon (:) between hours and minutes when writing time. They also add a letter to show the time zone (like "0340Z").
• They always say the leading zeros. So, 5:43 a.m. is spoken as "zero five forty-three."
• Time zones are given letters and special words from the NATO phonetic alphabet. For example, "0200R" is spoken as "zero two hundred Romeo."
• Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC) is called "Zulu time" and is shown with the letter Z.

The Story of Time: A Brief History

A unique 24-hour clock from 1443 in Florence Cathedral, Italy.

This 24-hour tower clock in Venice shows the hours 1 to 12 twice.

Long ago, in the 14th century, the first mechanical public clocks in Italy used a 24-hour system. These clocks counted 24 hours from about half an hour after sunset. The 24th hour was the last hour of daylight.

For several centuries, different ways of telling time competed in Europe. Some systems started the day after sunset, while others, like the German system, started at midnight. The modern 24-hour system we use today is based on this midnight-starting idea. It became popular around the world in the late 1800s.

Making Time Standard Worldwide

Sandford Fleming, a Canadian engineer, was a big supporter of using the 24-hour clock. He also wanted to create time zones and a standard starting line for measuring time around the world. In 1884, at a big meeting called the International Meridian Conference, countries agreed on an important rule:

That this universal day is to be a mean solar day; is to begin for all the world at the moment of midnight of the initial meridian coinciding with the beginning of the civil day and date of that meridian, and is to be counted from zero up to twenty-four hours.

This meant that the new "universal day" would start at midnight and count up to 24 hours.

How Countries Adopted the 24-Hour Clock

The Canadian Pacific Railway was one of the first big organizations to use the 24-hour clock in 1886. Italy was the first country to officially adopt it nationwide in 1893. Other European countries soon followed, like France in 1912, Denmark in 1916, and Greece in 1917. By the 1920s, many countries in Europe and Latin America had switched to the 24-hour system.

During World War I, the British Royal Navy started using the 24-hour clock in 1915. Other allied military forces quickly followed. The Canadian armed forces began using it in late 1917, and the British Army officially switched in 1918. In the United States, the Navy adopted the system in 1920, and the Army officially followed on July 1, 1942.

The Shepherd Gate Clock in Greenwich, England, shows hours up to XXIII (23) and 0 for midnight.

In the United Kingdom, the use of the 24-hour clock has slowly grown. In 1934, the British Broadcasting Corporation tried using it for TV and radio schedules, but people weren't very keen on it, so they went back to the 12-hour clock. Today, the BBC uses a mix of both. Public transport systems like British Rail and the London Underground switched to the 24-hour clock for their timetables in 1964. Many other English-speaking countries, like Canada, Australia, and New Zealand, also use a mix of both 12-hour and 24-hour clocks.

See also

In Spanish: Sistema horario de 24 horas para niños