Many wall clocks in American homes, schools, and businesses display the numbers 1–12. This is the same for many wristwatches. When telling time using these devices, people say numbers and then include the designator a.m. or p.m. (ante meridian or post meridian), depending on whether the time is before noon or after noon. This kind of timekeeping is done in a few other countries as well. However, most countries around the world—and, indeed the U.S. Military (and other military organizations)—use a 24-hour clock.

We already divide the day into 24 hours, each lasting 60 minutes, each of those lasting 60 seconds. Non-digital wall clocks have an hour hand, a minute hand, and a second hand. Digital wall clocks display the time by hour and minute(s) and sometimes include second(s). Some digital wall clocks display time on a scale of 0–24. This is true of many digital wristwatches as well.

Telling time using all 24 hours leads to what to some people would say as 1:00 p.m. being, to other people, 13:00. In the same way, 5:24 p.m. would be displayed, in the 24-hour time system, as 17:24 (because the numbers don't reset after 12 but continue on until midnight, which is usually 0 and not 24). The difference isn't noticeable for time before noon because 7:00 a.m. is referred to as 7:00.

The U.S. Military uses the 24-hour time system. Many Americans refer to this as Military Time.

Military Time has a few further differences:

• When saying time before 10:00, military personnel pronounce the zero. A time of 6:42 a.m. is pronounced "Zero six forty-two" or, more commonly "Oh six forty-two," with the "O" sound being substituted for "zero." The initial zero is also included in writing as well, so 9:30 a.m. becomes "0930."
• When the time is exactly on an hour, a person using Military Time will include the words "hundred hours" as part of the time. So 5:00 p.m. becomes "seventeen hundred hours." The inclusion of the word "hours" is a common one and sometimes applies to time when it is not exactly on an hour.
• A time of 10:00 a.m. in Military Time is "ten hundred hours." A time of 8:00 p.m. in Military Time is "twenty hundred hours." A time of midnight in Military Time is "zero hundred hours" (or sometimes "twenty-four hundred hours").
• Many notations of Military Time also include the Time Zone of the person telling the time. Military time zones around the world also have a letter designation, based on the NATO phonetic alphabet (Alpha, Bravo, Charlie, etc.). An example would be U.S. Eastern Standard Time and a time of 6:00 a.m. In Military Time, this would be written "0600R" and spoken "oh six hundred Romeo" because that time zone has the letter R assigned to it. In the same way, Greenwich Mean Time has the letter Z assigned to it, so noon in London would be referred to as "12 hundred hours Zulu." This system skips the letter J. Some instances of Military Time include the letter J when referring to the time where that person is: A person in New York, which is in the Eastern Standard Time Zone, would refer to 8:00 a.m. in Chicago, which is in the Central Standard Time Zone, as "0800S," since S is the NATO Alphabet letter assigned to that time zone. (Sierra is the NATO phonetic alphabet word assigned to the letter S.) However, a person in Chicago would refer to his or her own current time as "0800J," with the "J" signifying local time.

The U.S. Navy was the first branch of the U.S. Military to adopt a 24-hour time system, doing so in 1920. The U.S. Army followed suit but not until 1942. The U.S. Military briefly used the 24-hour time system during World War I.

Many other countries around the world use the 24-hour time system (but not the full Military Time treatment). The U.S. and Canada still officially use the 12-hour time system, as does the United Kingdom, although many transportation systems and broadcast entities in the U.K. use the 24-hour time system.

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