12-hour clock facts for kids

This map shows how the 12-hour or 24-hour clock is used around the world. Red areas mostly use 24-hour time. Green areas use both. Blue areas mostly use 12-hour time.

The 12-hour clock is a way to tell time that divides the 24 hours of a day into two periods. These periods are called a.m. (from Latin ante meridiem, meaning "before midday") and p.m. (from Latin post meridiem, meaning "after midday"). Each period has 12 hours, numbered: 12, then 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11.

This system for telling time has been used since ancient times. It reached its modern form around the 1500s. The 12-hour clock is very common in many English-speaking countries. This includes places like the United States, Canada, Australia, and the United Kingdom.

Understanding the 12-Hour Clock

How We Tell Time: A.M. and P.M.

The natural cycle of day and night helps us divide a calendar day. Originally, there was a cycle tracked by the sun (day) and another by the moon and stars (night). This idea grew into the two 12-hour periods we use today.

The "a.m." period starts at midnight and lasts until just before noon. For example, 7:00 a.m. is in the morning. The "p.m." period starts at noon and lasts until just before midnight. So, 7:00 p.m. is in the evening.

A Look Back: History of the 12-Hour Clock

The Exeter Cathedral Astronomical Clock shows a double-XII numbering system.

The 12-hour clock can be traced back thousands of years. Ancient civilizations like those in Mesopotamia and ancient Egypt used similar systems. For instance, a sundial for daytime and a water clock for nighttime were found in an Egyptian tomb from around c. 1500 BC. These clocks divided their use time into 12 hours each.

The Romans also used a 12-hour clock. They divided both daylight and nighttime into 12 equal parts. However, the length of these hours changed with the seasons. This was because daylight hours are longer in summer and shorter in winter.

The first mechanical clocks appeared in the 1300s. Some showed all 24 hours. But by the 1400s and 1500s, the 12-hour clock became the standard for public use in Northern Europe. Many old clocks, like those at Wells and Exeter, show both a.m. and p.m. hours on one dial. This is known as the double-XII system.

Most analog clocks and watches today use the 12-hour dial. The shorter hour hand rotates once every 12 hours. This means it goes around twice in one full day. Some analog clocks also have an inner ring of numbers. This ring shows numbers 13 through 23, allowing the clock to be read in 24-hour notation as well.

Where the 12-Hour Clock is Used

The 12-hour clock is the main way people write and speak about time in several countries. These are mostly nations that were once part of the British Empire. Examples include the United Kingdom, Republic of Ireland, the United States, Canada (except Quebec), Australia, New Zealand, and South Africa. Other countries like Mexico and the Philippines also follow this system.

Even in these countries, the 24-hour clock is often preferred in certain fields. This includes science, medicine, the military, or for transportation schedules. In most other countries, however, the 24-hour clock is the standard system. This is especially true in writing. Some nations in Europe and Latin America use a mix of both. They might speak using the 12-hour system but write using the 24-hour system.

When speaking, people often add phrases like "in the morning," "in the afternoon," "in the evening," and "at night." For example, "7:00 in the morning" or "9:00 at night."

Computer Time Settings

In most countries, computers usually show the time in 24-hour notation by default. However, many operating systems, like Microsoft Windows and Unix-like systems such as Linux and macOS, can be set to use the 12-hour notation. This often depends on the language and region settings chosen by the user. You can usually change this setting yourself.

Speaking About Time

Common Abbreviations: A.M. and P.M.

A typical digital clock showing p.m. with a dot.

A common analog 12-hour clock.

The Latin abbreviations a.m. and p.m. are used in English and Spanish. You might see them written in different ways. For example, "am" and "pm," "AM" and "PM," or "A.M." and "P.M."

Sometimes, if these abbreviations are not used, the meaning can still be clear. This depends on the situation and common habits. For example, if you say you start work at "9:00," it usually means 9:00 a.m. But if a party is scheduled for "9:00," it likely means 9:00 p.m.

Everyday Talk: Rounding and Phrases

When people talk about time, they often round it to the nearest five minutes. They might also say how many minutes "past" or "to" the closest hour it is.

• "Five past five" means 5:05.
• "Ten past five" means 5:10.
• "Ten to five" means 4:50 (ten minutes before 5:00).

Fifteen minutes is often called a "quarter hour." So, 5:15 can be phrased as "(a) quarter past five." Thirty minutes is often known as a "half hour." So, 5:30 can be "half past five."

Sometimes, the phrase "half five" is used to mean 4:30 instead of 5:30. This means "halfway to five." This way of speaking is common in some regions, especially where Germanic and Slavic languages have influenced how people talk.

Being Precise: Formal Time-Telling

For exact times, especially in formal speech, minutes are expressed as an exact number past the hour. For example, 6:32 p.m. is usually said as "six thirty-two p.m." It is also common to say "twenty-eight minutes to seven."

For full hours, people often use the word "o'clock." So, 10:00 is "ten o'clock." You can also just say "four a.m." or "four p.m." without "o'clock." When minutes are from ":01" to ":09," they are usually pronounced with an "oh." For instance, 6:02 a.m. is "six oh two a.m."

Avoiding Confusion: Noon and Midnight

The terms "12:00 a.m." and "12:00 p.m." can sometimes be confusing. This is because "a.m." means "before midday" and "p.m." means "after midday." Noon is exactly midday, and midnight is neither before nor after midday in the same way.

To avoid confusion, it is best to use clear terms:

• Use 12 noon for midday.
• Use 12 midnight for the middle of the night.

Many style guides and experts recommend using "12 noon" and "12 midnight." Another way to be very clear is to refer to one minute before or after 12:00. For example, "11:59 p.m." or "12:01 a.m." This is often used in legal documents and for travel schedules.

Historically, there have been different ways people used "12 a.m." and "12 p.m." Some older guides even used them in the opposite way we usually do today! This shows why using "noon" and "midnight" is much clearer.

See also

In Spanish: Sistema horario de 12 horas para niños