24-hour clock

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The modern 24-hour clock is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) passed since midnight, from 00(:00) to 23(:59), with 24(:00) as an option to indicate the end of the day. This system, as opposed to the 12-hour clock, is the most commonly used time notation in the world today, [A] and is used by the international standard ISO 8601. [1]

Contents

A 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 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. [2]

Description

24-hour clock 12-hour clock
00:0012:00 midnight [a]
12:00 a.m. (start of the day)
01:001:00 a.m.
02:002:00 a.m.
03:003:00 a.m.
04:004:00 a.m.
05:005:00 a.m.
06:006:00 a.m.
07:007:00 a.m.
08:008:00 a.m.
09:009:00 a.m.
10:0010:00 a.m.
11:0011:00 a.m.
12:0012:00 noon [a]
12:00 p.m.
13:001:00 p.m.
14:002:00 p.m.
15:003:00 p.m.
16:004:00 p.m.
17:005:00 p.m.
18:006:00 p.m.
19:007:00 p.m.
20:008:00 p.m.
21:009:00 p.m.
22:0010:00 p.m.
23:0011:00 p.m.
24:00 equals
00:00 next day		12:00 midnight [a]
(end of the day)
World map showing the usage of 12 or 24-hour clock in different countries .mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{} 24-hour 24-hour (12-hour orally) Both in common use 12-hour 12 24 Hours World Map.svg
World map showing the usage of 12 or 24-hour clock in different countries
  24-hour
  24-hour (12-hour orally)
  Both in common use
  12-hour
24-hour digital clock in Miaoli HSR station. Digital clock in THSR Miaoli Station.jpg
24-hour digital clock in Miaoli HSR station.
A public 24-hour clock in Curitiba, Brazil, with the hour hand on the outside and the minute hand on the inside. 24 hour analog clock rua 24 horas curitiba brasil.jpg
A public 24-hour clock in Curitiba, Brazil, with the hour hand on the outside and the minute hand on the inside.

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. To indicate the exact end of the day, hh may take the value 24, with mm and ss taking the value 00. 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 some computer protocols and military time), no separator is used and times are written as, for example, "2359".

Midnight 00:00 and 24:00

In the 24-hour time notation, the day begins at midnight, 00:00 or 0:00, 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 [3]  — 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. [4] Sometimes the use of 00:00 is also avoided. [4] In variance with this, as of 2010, the correspondence manual for the United States Navy and United States Marine Corps formerly specified 0001 to 2400. [5] The manual was updated in June 2015 to use 0000 to 2359. [6]

Times after 24:00

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 United Kingdom, France, Spain, Canada, Japan, South Korea, Hong Kong, and China where business hours extend beyond midnight, such as broadcast television production and scheduling. The GTFS public transport schedule listings file format has the concept of service days and expects times beyond 24:00 for trips that run after midnight. [7]

Computer support

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. POSIX programs such as ls default to displaying timestamps in 24-hour format.

Military time

24-hour clock as seen on the USS Midway. 24-Hour Clock Aboard the USS Midway (8727193240).jpg
24-hour clock as seen on the USS Midway.

In American English, the term military time is a synonym for the 24-hour clock. [8] In the US, 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, [9] differs in some respects from other twenty-four-hour time systems:

History

Paolo Uccello's Face with Four Prophets/Evangelists (1443) in Florence Cathedral Florence-Duomo-Clock.jpg
Paolo Uccello's Face with Four Prophets/Evangelists (1443) in Florence Cathedral
The 24-hour tower clock in Venice that lists hours 1 to 12 twice Clock 24 J.jpg
The 24-hour tower clock in Venice that lists hours 1 to 12 twice

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 sunset to the evening of the following day. The 24th hour was the last hour of day time. [11]

From the 14th to the 17th century, two systems of time measurement competed in Europe: [12] [13]

The modern 24-hour system is a late-19th century adaptation of the German midnight-starting system, and then prevailed in the world with the exception of some Anglophone countries.

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 four times (84 strokes). [11]

Sandford Fleming, the engineer-in-chief of the Canadian Intercolonial Railway, 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. [15] [16] At the International Meridian Conference in 1884, the following resolution was adopted by the conference: [17]

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. [17]

The Shepherd Gate Clock with Roman numerals up to XXIII (23) and 0 for midnight, in Greenwich Greenwich clock 1-manipulated.jpg
The Shepherd Gate Clock with Roman numerals up to XXIII (23) and 0 for midnight, in Greenwich

The Canadian Pacific Railway was among the first organisations to adopt the 24-hour clock, at midsummer 1886. [15] [18] 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. [19] 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. [20] Some of the railways in India had switched before the outbreak of the war. [19]

During World War I, the British Royal Navy adopted the 24-hour clock in 1915, and the Allied armed forces followed soon after, [19] with the British Army switching officially in 1918. [21] The Canadian armed forces first started to use the 24-hour clock in late 1917. [22] In 1920, the United States Navy was the first United States organisation to adopt the system; the United States Army, however, did not officially adopt the 24-hour clock until 1 July 1942. [23] [24]

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. [25] In 1934, the British Broadcasting Corporation (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. [25] In the same year, Pan American World Airways Corporation and Western Airlines in the United States both adopted the 24-hour clock. [26] In modern times, the BBC uses a mixture of both the 12-hour and the 24-hour clock. [25] British Rail, London Transport, and the London Underground switched to the 24-hour clock for timetables in 1964. [25] 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 as well as New Zealand also use both systems.

See also

Notes

  1. ↑ See the Common Locale Data Repository for detailed data about the preferred date and time notations used across the world, as well the locale settings of major computer operating systems, and the article Date and time representation by country.

Related Research Articles

A calendar date is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "25 December 2024" is ten days after "15 December 2024". The date of a particular event depends on the observed time zone. For example, the air attack on Pearl Harbor that began at 7:48 a.m. Hawaiian time on 7 December 1941 took place at 3:18 a.m. Japan Standard Time, 8 December in Japan.

<span class="mw-page-title-main">Greenwich Mean Time</span> Time zone of Western Europe, same as WET

Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

<span class="mw-page-title-main">ISO 8601</span> International standards for dates and times

ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the International Organization for Standardization (ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019, and an amendment in 2022. The standard provides a well-defined, unambiguous method of representing calendar dates and times in worldwide communications, especially to avoid misinterpreting numeric dates and times when such data is transferred between countries with different conventions for writing numeric dates and times.

<span class="mw-page-title-main">Time zone</span> Area that observes a uniform standard time

A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

<span class="mw-page-title-main">International Meridian Conference</span> 1884 conference in Washington, D.C., United States

The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use. The conference was held at the request of U.S. President Chester A. Arthur. The subject to discuss was the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world". It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude.

The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. and p.m.. Each period consists of 12 hours numbered: 12, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 12-hour clock has been developed since the second millennium BC and reached its modern form in the 16th century.

<span class="mw-page-title-main">Midnight</span> Transition time from one day to the next

Midnight is the transition time from one day to the next – the moment when the date changes, on the local official clock time for any particular jurisdiction. By clock time, midnight is the opposite of noon, differing from it by 12 hours.

Different conventions exist around the world for date and time representation, both written and spoken.

Date and time notation in Canada combines conventions from the United Kingdom, conventions from the United States, and conventions from France, often creating confusion. The Government of Canada specifies the ISO 8601 format for all-numeric dates. It recommends writing the time using the 24-hour clock (17:10) for maximum clarity in both Canadian English and Canadian French, but also allows the 12-hour clock (5:10 p.m.) in English.

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 Poland, the official system for representing dates and times follows the international ISO 8601 standard. However, in less official use, other conventions prevail, such as the day-month-year order and several Polish language abbreviations.

Date and time notation in Sweden mostly follows the ISO 8601 standard: dates are generally written in the form YYYY-MM-DD. Although this format may be abbreviated in a number of ways, almost all Swedish date notations state the month between the year and the day. Months are not capitalised when written. The week number may also be used in writing and in speech. Times are generally written using 24-hour clock notation, with full stops as separators, although 12-hour clock notation is more frequently used in speech.

Date and time notation in the United Kingdom records the date using the day–month–year format. The time can be written using either the 24-hour clock (23:59) or the 12-hour clock (11:59 p.m.), either with a colon or a full stop (11.59 p.m.).

The European Committee for Standardization (CEN) and (CENELEC) adopted ISO 8601 with EN 28601, now EN ISO 8601. As a European Norm, CEN and CENELEC member states are obligated to adopt the standard as national standard without alterations as well.

Thailand has adopted ISO 8601 under national standard: TIS 1111:2535 in 1992. However, Thai date and time notation reflects the country’s cultural development through the years used. The formal date format is D/M/YYYY format (1/6/2568), nowadays using the Buddhist Era (BE). The full date format is day-month-year format which is written in Thai. While a 24-hour system is common for official use, colloquially, a 12-hour format with terms like "morning" and "night", etc., or a modified six-hour format is used.

<span class="mw-page-title-main">Date and time notation in the Philippines</span>

Date and time notation in the Philippines varies across the country in various, customary formats. Some government agencies in the Philippines have adopted time and date representation standard based on the ISO 8601, notably the Philippines driver's license and the Unified Multi-Purpose ID.

<span class="mw-page-title-main">Time in Finland</span>

Finland uses Eastern European Time (EET) during the winter as standard time and Eastern European Summer Time (EEST) during the summer as daylight saving time. EET is two hours ahead of coordinated universal time (UTC+02:00) and EEST is three hours ahead of coordinated universal time (UTC+03:00). Finland adopted EET on 30 April 1921, and has observed daylight saving time in its current alignment since 1981 by advancing the clock forward one hour at 03:00 EET on the last Sunday in March and back at 04:00 EET on the last Sunday in October, doing so an hour earlier for the first two years.

Date and time notation in New Zealand most commonly records the date using the day-month-year format, while the ISO 8601 format (2024-12-14) is increasingly used for all-numeric dates, such as date of birth. The time can be written using either the 12-hour clock or the 24-hour clock (12:38).

References

  1. ↑ ISO 8601:2004 – Data elements and interchange formats – Information interchange – Representation of dates and times (3rd ed.). 2004.
  2. ↑ Pickar, Gloria D.; Graham, Hope; Swart, Beth; Swedish, Margaret (2011). Dosage calculations (2nd Canadian ed.). Toronto: Nelson Education. p. 60. ISBN   9780176502591.
  3. ↑ ISO 8601:2004 – Data elements and interchange formats – Information interchange – Representation of dates and times (3rd ed.). 2004. Clause 4.2.3 "Midnight".
  4. 1 2 Communication instructions – General (PDF). Combined Communications-Electronics Board. October 2010. p. 3-6. ACP 121(I). Archived from the original (PDF) on 7 August 2011.
  5. ↑ Department of the Navy Correspondence Manual (PDF). United States Department of the Navy. March 2010. Chapter 2, Section 5 Paragraph 15. Expressing Military Time. SECNAV M-5216.5.
  6. ↑ Department of the Navy Correspondence Manual (PDF). United States Department of the Navy. June 2015. Chapter 2, Section 5 Paragraph 15. Expressing Military Time. SECNAV M-5216.5.
  7. ↑ "Reference | Static Transit". Google Developers.
  8. ↑ "military time". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 12 November 2020.
  9. ↑ "Communication Instructions General ACP 121(I)" (PDF). Archived from the original (PDF) on 8 May 2016.
  10. ↑ Guinot, Bernard (August 2011). "Solar time, legal time, time in use". Metrologia. 48 (4): 185. Bibcode:2011Metro..48S.181G. doi:10.1088/0026-1394/48/4/S08. S2CID   121852011.
  11. 1 2 Dohrn-Van Rossum, Gerhard (1996). History of the Hour. Clock and Modern Temporal Orders. The University of Chicago Press. p. 114. ISBN   0226155110.
  12. ↑ Tomáš Voříšek: Hodiny německé a české na půl orloje a podle celého orloje, Otta z Losu, last update 2021.
  13. ↑ Stan. Marušák, Petr Král: Čtyřiadvacetník, Orloj.eu.
  14. ↑ Český čas s otazníkem, Orloj.eu.
  15. 1 2 Fleming, Sandford (1886). "Time-reckoning for the twentieth century". Annual Report of the Board of Regents of the Smithsonian Institution (1): 345–366. Reprinted in 1889: Time-reckoning for the twentieth century at the Internet Archive .
  16. ↑ Creet, Mario (1990). "Sandford Fleming and Universal Time". Scientia Canadensis: Canadian Journal of the History of Science, Technology and Medicine. 14 (1–2): 66–89. doi: 10.7202/800302ar .
  17. 1 2 "International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October 1884. Protocols of the proceedings". Project Gutenberg. 1884. Retrieved 30 November 2012.
  18. ↑ The London Times reports on a timetable using the 24-hour clock on a trip from Port Arthur, Ontario: "A Canadian Tour". The Times. No. 31880. London. 2 October 1886. col 1–2, p. 8.
  19. 1 2 3 "Memorandum CAB 24/110/21 (CP 1721), 'Report of the Committee upon the 24 hour method of expressing time'". The National Archives, Kew, United Kingdom. 4 August 1920.
  20. ↑ "History of the 24-Hour Clock - 24 Hour Time Clock". Military Time Chart. 24 December 2016. Retrieved 27 July 2023.
  21. ↑ The Times : 1918 September 19, p. 3.
  22. ↑ Dancocks, Daniel G. Gallant Canadians: The Story of the 10th Canadian Infantry Battalion 1914–1919.
  23. ↑ "Army Adopts 24-Hour Clock Plan". Brownsville Herald. Brownsville, Texas. 26 June 1942. p. 2. Effective July 1, the United States Army will adopt the 24-hour clock system for all messages, dispatches, orders and reports, the War Department has announced. Under this new system time will be expressed in a group of four digits ranging from 0000 to 2400. [...] Flying officers at Moore Field have used the 24-hour clock system for some time. However, its use in official business messages will be an innovation.
  24. ↑ "It Will Be 2100 O'Clock at Ft. Bliss At 3 p. m." El Paso Herald-Post. El Paso. 3 July 1942. p. 3. The Army has a new time system — Greenwich Civil Time — to be used in radiograms, telegrams, and all official communications. [...] Effective 10 p. m. Monday, the War Department adopted the 24-hour system, which is used internationally with the Navy and the Armed forces of associated nations. [...] the six hours difference between Mountain War Time and actual Greenwich time must be taken into account, so that 12 noon is expressed on communications as 1800.
  25. 1 2 3 4 Boardman, Peter (July 2011). Counting Time: a brief history of the 24-hour clock.[ self-published source ]
  26. ↑ Sarasota Herald-Tribune 1943 May 14.
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