The 24-hour clock is the convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today, and is used by international standard ISO 8601.
A limited number of countries, particularly English-speaking, use the 12-hour clock, or a mixture of the 24- and 12-hour time systems. In countries where the 12-hour clock is still dominant, some professions prefer to use the 24-hour clock. For example, in the practice of medicine the 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patient's medical history. In the United States and a handful of other countries, it is popularly referred to as military time.
|24-hour clock||12-hour clock|
(start of the day)
|12:00||12:00 noon *|
(end of the day)
|* See "Confusion at noon and midnight"|
A time of day is written in the 24-hour notation in the form hh:mm (for example 01:23) or hh:mm:ss (for example, 01:23:45), where hh (00 to 23) is the number of full hours that have passed since midnight, mm (00 to 59) is the number of full minutes that have passed since the last full hour, and ss (00 to 59) is the number of seconds since the last full minute. In the case of a leap second, the value of ss may extend to 60. A leading zero is added for numbers under 10, but it is optional for the hours. The leading zero is very commonly used in computer applications, and always used when a specification requires it (for example, ISO 8601).
Where subsecond resolution is required, the seconds can be a decimal fraction; that is, the fractional part follows a decimal dot or comma, as in 01:23:45.678. The most commonly used separator symbol between hours, minutes and seconds is the colon, which is also the symbol used in ISO 8601. In the past, some European countries used the dot on the line as a separator, but most national standards on time notation have since then been changed to the international standard colon. In some contexts (including the U.S. military and some computer protocols), no separator is used and times are written as, for example, "2359".
In the 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day begins at 23:59. Where convenient, the notation 24:00 may also be used to refer to midnight at the end of a given date – that is, 24:00 of one day is the same time as 00:00 of the following day.
The notation 24:00 mainly serves to refer to the exact end of a day in a time interval. A typical usage is giving opening hours ending at midnight (e.g. "00:00–24:00", "07:00–24:00"). Similarly, some bus and train timetables show 00:00 as departure time and 24:00 as arrival time. Legal contracts often run from the start date at 00:00 until the end date at 24:00.
While the 24-hour notation unambiguously distinguishes between midnight at the start (00:00) and end (24:00) of any given date, there is no commonly accepted distinction among users of the 12-hour notation. Style guides and military communication regulations in some English-speaking countries discourage the use of 24:00 even in the 24-hour notation, and recommend reporting times near midnight as 23:59 or 00:01 instead. Sometimes the use of 00:00 is also avoided. In variance with this, the correspondence manual for the U.S. Navy and U.S. Marine Corps formerly specified 0001 to 2400. The manual was updated in June 2015 to use 0000 to 2359.
Time-of-day notations beyond 24:00 (such as 24:01 or 25:00 instead of 00:01 or 01:00) are not commonly used and not covered by the relevant standards. However, they have been used occasionally in some special contexts in the UK, Canada, Japan, South Korea, Hong Kong and China where business hours extend beyond midnight, such as broadcast television production and scheduling.
In most countries, computers by default show the time in 24-hour notation. For example, Microsoft Windows and macOS activate the 12-hour notation by default only if a computer is in a handful of specific language and region settings. The 24-hour system is commonly used in text-based interfaces. Programs such as ls default to displaying timestamps in 24 hour format.
In American and Canadian English, the term military time is a synonym for the 24-hour clock. In these dialects, the time of day is customarily given almost exclusively using the 12-hour clock notation, which counts the hours of the day as 12, 1, ..., 11 with suffixes a.m. and p.m. distinguishing the two diurnal repetitions of this sequence. The 24-hour clock is commonly used there only in some specialist areas (military, aviation, navigation, tourism, meteorology, astronomy, computing, logistics, emergency services, hospitals), where the ambiguities of the 12-hour notation are deemed too inconvenient, cumbersome, or dangerous.
Military usage, as agreed between the United States and allied English-speaking military forces, differs in some respects from other twenty-four-hour time systems:
The 24-hour time system has its origins in the Egyptian astronomical system of decans, and has been used for centuries by scientists, astronomers, navigators, and horologists. In East Asia, time notation was 24-hour before westernization in modern times. Western-made clocks were changed into 12 dual-hours style when they were shipped to China in the Qing dynasty. There are many surviving examples of clocks built using the 24-hour system, including the famous Orloj in Prague, and the Shepherd Gate Clock at Greenwich.
The first mechanical public clocks introduced in Italy were mechanical 24-hour clocks which counted the 24 hours of the day from one half hour after sundown to the evening of the following day. The 24th hour was the last hour of day time. However, striking clocks had to produce 300 strokes each day which required a lot of rope, and wore out the mechanism quickly, so some localities switched to ringing sequences of 1 to 12 twice (156 strokes), or even 1 to 6 repeated 4 times (84 strokes).
After missing a train while travelling in Ireland in 1876 because a printed schedule listed p.m. instead of a.m., Sir Sandford Fleming proposed a single 24-hour clock for the entire world, located at the centre of the Earth, not linked to any surface meridian – a predecessor to Coordinated Universal Time. He was an early proponent of using the 24-hour clock as part of a programme to reform timekeeping, which also included establishing time zones and a standard prime meridian. The Canadian Pacific Railway was among the first organizations to adopt the 24-hour clock, at midsummer 1886.
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 resolution was adopted by the conference.
A report by a government committee in the United Kingdom noted Italy as the first country among those mentioned to adopt 24-hour time nationally, in 1893. Other European countries followed: France adopted it in 1912 (the French army in 1909), followed by Denmark (1916), and Greece (1917). By 1920, Spain, Portugal, Belgium, and Switzerland had switched, followed by Turkey (1925), and Germany (1927). By the early 1920s, many countries in Latin America had also adopted the 24-hour clock. Some of the railways in India had switched before the outbreak of the war.
During World War I, the British Royal Navy adopted the 24-hour clock in 1915, and the Allied armed forces followed soon after, with the British Army switching officially in 1918. The Canadian armed forces first started to use the 24-hour clock in late 1917. In 1920, the US Navy was the first US organization to adopt the system; the US Army, however, did not officially adopt the 24-hour clock until World War II, on July 1, 1942.
The use of the 24-hour clock in the United Kingdom has grown steadily since the beginning of the 20th century, although attempts to make the system official failed more than once. In 1934, the BBC switched to the 24-hour clock for broadcast announcements and programme listings. The experiment was halted after five months following a lack of enthusiasm from the public, and the BBC continued using the 12-hour clock. In the same year, the US airlines Pan American World Airways Corporation and Western Airlines both adopted the 24-hour clock. In modern times, the BBC uses a mixture of both the 12-hour and the 24-hour clock. British Rail and London Transport switched to the 24-hour clock for timetables in 1964. A mixture of the 12- and 24-hour clocks similarly prevails in other English-speaking Commonwealth countries: French speakers have adopted the 24-hour clock in Canada much more broadly than English speakers, and Australia also uses both systems.
The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translates to, before midday) and p.m. (from Latin post meridiem translates to, past midday). Each period consists of 12 hours numbered: 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 24 hour/day cycle starts at 12 midnight (may be indicated as 12 a.m.), runs through 12 noon (may be indicated as 12 p.m.), and continues to the midnight at the end of the day. The 12-hour clock has been developed from the middle of the second millennium BC to the 16th century AD.
The 12-hour time convention is common in several English-speaking nations and former British colonies, as well as a few other countries.24-hour analog dial
Clocks and watches with a 24-hour analog dial have an hour hand that makes one complete revolution, 360°, in a day (24 hours per revolution). The more familiar 12-hour analog dial has an hour hand that makes two complete revolutions in a day (12 hours per revolution).
Twenty-four-hour analog clocks and watches are used today by pilots, scientists, and the military, and are sometimes preferred because of the unambiguous representation of a whole day at a time. Note that this definition refers to the use of a complete circular dial to represent a 24-hour day. Using the numbers from 0 to 23 (or 1 to 24) to mark the day is the 24-hour clock system.
Sundials use 24-hour analog dials—the shadow traces a path that repeats approximately once per day.
Many sundials are marked with the double-XII or double-12 system, in which the numbers I to XII (or 1 to 12) are used twice, once for the morning hours, and once for the afternoon and evening hours. So VI (or 6) appears twice on many dials, once near sunrise and once near sunset.
Modern 24-hour analog dials—other than sundials—are almost always marked with 24 numbers or hour marks around the edge, using the 24-hour clock system. These dials do not need to indicate AM or PM.Continental time
In the United Kingdom continental time can refer to the Central European Time zone or to the 24-hour clock, though the latter usage has been declining as use of the 24-hour clock has increased.Date and time notation in Australia
Date and time notation in Australia largely follows conventions from British English, like many other aspects of Australian English. It most commonly records the date using the day-month-year format (22 April 2019), while the ISO 8601 format (2019-04-22) is increasingly used for all-numeric dates. The time can be written using either the 24-hour clock (18:22) or 12-hour clock (6:22 pm).Date and time notation in Canada
Date and time notation in Canada combines conventions from the United Kingdom, the United States, and France, often creating confusion. The Government of Canada specifies the ISO 8601 format for all-numeric dates (YYYY-MM-DD; for example, 2019-04-20). It recommends writing the time using the 24-hour clock (17:35) for maximum clarity in both English and French, but also allows the 12-hour clock (5:35 p.m.) in English.Date and time notation in Turkey
In Turkey, the little endian date format is used, and 24-hour clock is more common than 12-hour clock system.Date and time notation in the United Kingdom
Date and time notation in the United Kingdom records the date using the day-month-year format (21 October 2011 or 21/10/11). The ISO 8601 format (2011-10-21) is increasingly used for all-numeric dates. The time can be written using either the 24-hour clock (16:10) or 12-hour clock (4.10 p.m.).Date and time notation in the United States
Date and time notation in the United States differs from that used in nearly all other countries. It is inherited from one historical branch of conventions from the United Kingdom. American styles of notation have also influenced customs of date notation in Canada, creating confusion in international commerce.In traditional American usage, dates are written in the month–day–year order (e.g. April 22, 2019) with a comma before and after the year if it is not at the end of a sentence, and time in 12-hour notation (2:12 pm).
International date and time formats typically follow the ISO 8601 format (2019-04-22) for all-numeric dates, write the time using the 24-hour clock (14:12), and notate the date using a day–month–year format (22 April 2019). These forms are increasingly common in American professional, academic, technological, military, and other internationally oriented environments.Date and time representation by country
Different conventions exist around the world for date and time representation, both written and spoken.Hours (disambiguation)
Hours may refer to:
The plural of the unit of time, hour
an expression of time using the 24-hour clock system (e.g. "1300 hours")Italian six-hour clock
The six-hour clock (Italian: sistema orario a 6 ore), also called the Roman (alla romana) or the Italian (all'italiana) system, is a timekeeping system used in Italy. In this system, the day starts at the evening Ave Maria at the end of twilight, approximately half an hour after sunset, and the following 24 hours are divided into four cycles of six hours each.
Introduced by the Catholic Church in the 13th century, it remained in use in Italy until superseded by the 12-hour clock following the Napoleonic invasion of Italy.Midnight
Midnight is the transition time from one day to the next – the moment when the date changes. In ancient Roman timekeeping, midnight was halfway between sunset and sunrise (i.e., solar midnight), varying according to the seasons. By clock time, midnight is the opposite of noon, differing from it by 12 hours.
Solar midnight is the time opposite to solar noon, when the Sun is closest to the nadir, and the night is equidistant from dusk and dawn. Due to the advent of time zones, which regularize time across a range of meridians, and daylight saving time, it rarely coincides with 12 midnight on the clock. Solar midnight depends on longitude and time of the year rather than on time zone.
In the Northern Hemisphere, "midnight" had an ancient geographic association with "north" (as did "noon" with "south" – see noon). Modern Polish, Belarusian, Ukrainian and Serbian preserve this association with its word for "midnight" (północ, поўнач, північ, пoнoħ – literally "half-night"), which also means "north".Sandford Fleming
Sir Sandford Fleming (January 7, 1827 – July 22, 1915) was a Scottish Canadian engineer and inventor. Born and raised in Scotland, he emigrated to colonial Canada at the age of 18. He promoted worldwide standard time zones, a prime meridian, and use of the 24-hour clock as key elements to communicating the accurate time, all of which influenced the creation of Coordinated Universal Time. He designed Canada's first postage stamp, left a huge body of surveying and map making, engineered much of the Intercolonial Railway and the Canadian Pacific Railway, and was a founding member of the Royal Society of Canada and founder of the Canadian Institute, a science organization in Toronto.Thai six-hour clock
The six-hour clock is a traditional timekeeping system used in the Thai and formerly the Lao language and the Khmer language, alongside the official 24-hour clock. Like other common systems, it counts twenty-four hours in a day, but divides the day into four quarters, counting six hours in each. The hours in each quarter (with the exception of the sixth hour in each quarter) are told with period-designating words or phrases, which are:
... mong chao (Thai: ...โมงเช้า, [mōːŋ tɕʰáːw]) for the first half of daytime (07:00 to 12:59)
Bai ... mong (บ่าย...โมง, [bàːj mōːŋ]) for the latter half of daytime (13:00 to 18:59)
... thum (...ทุ่ม, [tʰûm]) for the first half of nighttime (19:00 to 00:59)
Ti ... (ตี..., [tīː]) for the latter half of nighttime (01:00 to 06:59)These terms are thought to have originated from the sounds of traditional timekeeping devices. The gong was used to announce the hours in daytime, and the drum at night. Hence the terms mong, an onomatopoeia of the sound of the gong, and thum, that of the sound of the drum. Ti is a verb meaning to hit or strike, and is presumed to have originated from the act of striking the timekeeping device itself. Chao and bai translate as morning and afternoon respectively, and help to differentiate the two daytime quarters.
The sixth hours of each quarter are told by a different set of terms. The sixth hour at dawn is called yam rung (ย่ำรุ่ง, [jâm rûŋ]), and the sixth hour at dusk is called yam kham (ย่ำค่ำ, [jâm kʰâm]), both references to the act of striking the gong or drum in succession to announce the turning of day (yam), where rung and kham, meaning dawn and dusk, denote the time of these occurrences. The midday and midnight hours are respectively known as thiang (เที่ยง, [tʰîaːŋ], or thiang wan, เที่ยงวัน, [tʰîaːŋ wān]) and thiang khuen (เที่ยงคืน, [tʰîaːŋ kʰɯ̄ːn]), both of which literally translate as midday and midnight.Midnight is also called song yam (สองยาม, [sɔ̌ːŋ jāːm]; note that yam is a different word), a reference to the end of the second three-hour period of the night watch (song translates as the number two). In addition, hok (6) thum and ti hok may also be used to refer to the hours of midnight and dawn, following general usage for the other hours, although more rarely; and the fourth to sixth hours of the second daytime half may also be told as ...mong yen (...โมงเย็น, [mōːŋ jēn]), yen meaning evening.
The system has been used in some form since the days of the Ayutthaya Kingdom, but was codified similarly to its present form only in 1901 by King Chulalongkorn in Royal Gazette 17:206. Nowadays, it is used only in colloquial speech. However, a corrupted form of the six-hour clock is more frequently encountered, where usually the first half of daytime (including the sixth hour of the preceding quarter) is counted as in the twelve-hour clock, i.e. hok (6) mong chao, chet (7) mong, etc., up to sip et (11) mong.
The six-hour clock system was abolished in Laos and Cambodia during the French protectorate, and the French 24-hour clock system (for example, 3h00) has been used since.
A comparison of the systems is as follows:
* The word chao (เช้า) is optional here since the numbers 7 to 11 are not used elsewhere
** Conversationally, si mong yen (สี่โมงเย็น) and ha mong yen (ห้าโมงเย็น) are also spoken if considered as eveningTime in Canada
Canada is divided into six time zones, based on proposals by Scottish Canadian railway engineer Sir Sandford Fleming, who pioneered the use of the 24-hour clock, the world's time zone system, and a standard prime meridian. Most of Canada operates on standard time from the first Sunday in November to the second Sunday in March and daylight saving time the rest of the year.
|Philosophy of time|
and use of time
|Time in physics|
|Archaeology and geology|
|Other units of time|