The 12-hour and 24-hour Clock System

The 12-hour and 24-hour Clock Systems

Background Information

Early Egyptians were the first to use a 12-hour clock system that divided the day into two periods: dusk to dawn and dawn to dusk. Early Romans also used a 12-hour clock that divided day and night into 12 equal hours.

For a long time, people used 12-hour clocks with a long hand to keep track of the hours and a short hand to keep track of the minutes. The biggest problem with that type of timekeeping device was that sometimes it was unclear whether the clock as referring to time before noon or after noon. For example, did one person another to meet at 6 on a 12-hour clock mean 6 in the morning or 6 in the evening? And did 12 refer to noon midnight? In the 15th century, people started using the term o’clock, meaning “of the clock.” For example, someone would say, “Come at seven o’clock in the morning” or “Let’s go at 11 o’clock at night.” In the 16th century, as more people write the time, they devised a faster and simpler way of indicating daytime or night time, by adding a.m., ante meridiem, Latin for before noon, or p.m., post meridiem, Latin for after noon. Another problem with the 12-hour clock was that it did not always show the passage of time accurately. When early Egyptians and Romans adopted the 12-hour clock, they divided daytime hours by 12 ad nighttime hours by 12. In their part of the world, near the equator, the daylight and nighttime hours were about the same all year round, so the length of an hour varied only slightly from season to season. However, this system did not work well for people living further from the equator, where daylight might last longer in the summer than the winter.

With the introduction of mechanical clocks in the 1600s, knowing time accurately became possible and expected. By the 1800s many countries in the world had started using the 24-hour clock system, which tells time by dividing the ay into 24 equal hours.

The 24-hour clock system originated with the ancient Babylonians, whose numeric system was based on the number 60. Under the 24-hour clock system, 1:00refers to the first hour of the day, 14:00 to the fourteenth hour of the day, and 24:00 to the twenty-fourth hour of the day, midnight. Midnight can also e expressed as 0:00, the start of the first hour of the day. With this system it is easy to calculate the passage of time (e.g., trade times of 11:00 to 14:00 refer to a trip taken in the midday hours).

The 24-hour clock system appeals to economic activities that rely on precision and accuracy and operate 24 hours a day. In some countries, the 24-hour clock system was called the military system because the military were among the first ti adopt it. Now the 24-hour clock system is used in travel, computers, digital clocks and watches, factories, industries, businesses, shipping, and many other activities. Its simplicity has made it the system that most countries of the world today use to tell time. In some countries such as the US and Canada, people need to be familiar with the 12-hour and the 24-hour systems in their everyday lives, especially when calculating arrival and departure times for trips by air, train, and bus.

Activity 1: Calculating Time Using 24-hour and 12-hour Clocks

Purpose: To practice telling time using the 24-hour clock system


  • Chart showing comparison ofttimes between the 24-hour clock system and the 12-hour clock system
  • Clock or watch
  • World History journals and pencils


Most Montessori teachers present this concept in Year 4 and present it in more details in Years 5 & 6

Announce to the students will have an opportunity to tell time using two different methods.


Invite the students to look at the classroom clock or their watches to tell what time it is.

Ask the students to look closely at how their watches and the clock tell time. Do they show time alone, or do they also showtime of the day or night?

Invite the students to imagine making an appointment fr a meeting next week at 8:15. Discuss whether the students could tell by this time alone whether the meeting was in the early morning or in the evening.

Define and discuss the 12-hour clock system, a.m. and p.m., and the o’clock. Using the example of 8:15 again, discuss why it might be necessary to the numbest add information tot he numbers to indicate time of day.

Define and discuss the 24-hour clock system. Using the example of 8:15 again, discuss why the 24-hour clock system has proven so useful for military, travel, and business purposes.

Referring to the chart, invite the students to convert a given time on the 24-hour clock system to a time n the 12-hour clock system. Repeat, sometimes reversing the exercise the exercise so the students convert from the 12-hour clock system to the 24-hour clock system.

Place the chart whee the students can easily access it.

Encourage the students to continue practicing converting ties from one system to another.

Ask the students to use their journals and the chart to answer several questions like this: You want leave for a nearby city, which ou know is four hours by train at eight in the morning so that you will arrive at noon. Write down the 12-hour clock system of expressing these times, then write down the 24-hour system you will look for on the train schedule.

Years 5 & 6

Review the 24-hour clock system.

Invite the students to convert given times expressed in words to numbers expressed by the 24-hour clock system.

Ask the students house their journals to create a chart summarizing the advantages and disadvantages of the 12-hour system, the 24-hour system and time expressed as words.


Visit a local travel bureau, airport, bus station, or translation and not scheduled arrival and departure times for given locations.

Obtain a train or bus schedule and calculate departure and arrival times for a trip on a given day from local town or city to a town or city nearby.

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