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24-hour clock

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The24-hour clockis the convention of time keeping in which thedayruns from midnight to midnight and is divided into 24hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today,[1]and is used by international standardISO 8601.[2]

A limited number of countries, particularly English-speaking nations, use the12-hour clockas a standard, 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 ofmedicinethe 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patientsmedical history.[3]In the United States and a handful of other countries, it is popularly referred to asmilitary time.[1][4]

A Russian 24 hour watch for polar expeditions from 1969, made by Soviet watchmakerRaketa. Polar nights or days make it necessary to use a 24-hour scale instead of 12.

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 sincemidnight, 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 aleap 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 specifications require it (for example,ISO 8601).

Where subsecond resolution is required, the seconds can be adecimal 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 thecolon, which is also the symbol used in ISO 8601. In the past, some European countries used thedot on the lineas 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.

InEast 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 toChinain theQing dynasty.

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[5] 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. 00:0024:00, 07:0024:00). Similarly, some railway 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 guidesand 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.[6]Sometimes the use of 00:00 is also avoided.[6]In variance with this, the correspondence manual for the U.S. Navy and U.S. Marine Corps formerly specified 0001 to 2400.[7]The manual was updated in June 2015 to use 0000 to 2359.[8]

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, 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 WindowsandmacOSactivate the 12-hour notation by default only if a computer is in a handful ofspecific language and region settings. The 24-hour system is commonly used in text-based interfaces. Programs such aslsdefault to displaying timestamps in 24 hour format.

In American and Canadian English, the termmilitary timeis a synonym for the 24-hour clock.[9]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 suffixesa.m.andp.m.distinguishing the twodiurnalrepetitions 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 theambiguities of the 12-hour notationare deemed too inconvenient, cumbersome, or dangerous.

Military usage, as agreed between the United States and allied English-speaking military forces,[10]differs in some respects from other twenty-four-hour time systems:

No hours/minutes separator is used when writing the time, and a letter designating the time zone is appended (for example 0340Z).

Leading zerosare always written out and are required to be spoken, so 5:43a.m. is spoken zero five forty-three (casually) or zero five four three (military radio), as opposed to five forty-three or five four three.

Military time zonesare lettered and thus given word designations via theNATO phonetic alphabet. For example, Eastern Standard Time(UTC−5) would be written 0600R and spoken zero six hundred Romeo.

Local time is designated as zone J orJuliett. 1200J (twelve hundred Juliett) is noon local time.

Greenwich Mean Time(orCoordinated Universal Time) is designated time zone Z, and thus called Zulu time.

Hours are always hundred, never thousand; 1000 is ten hundred not one thousand; 2000 is twenty hundred not two thousand.

Face with Four Prophets/Evangelists

The 24 hour tower clock inVenicethat lists hours 1 to 12 twice

The 24-hour time system has its origins in the Egyptian astronomical system ofdecans, and has been used for centuries by scientists, astronomers, navigators, andhorologists. There are many surviving examples of clocks built using the 24-hour system, including the famousOrlojinPrague, and theShepherd Gate ClockatGreenwich.[citation needed]

The first mechanical public clocks introduced in Italy weremechanical 24-hour clockswhich 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.[11]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).[11]

At theInternational Meridian Conferencein 1884,Lewis M. Rutherfurdproposed that:[12]

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.[12]

According to a report in the LondonTimesin 1886, the 24-hour clock was in use on theCanadian Pacific Railwaytrain at Port Arthur.[13]

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.[14]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.[citation needed]Some of the railways in India had switched before the outbreak of the war.[14]

DuringWorld War I, the British Royal Navy adopted the 24-hour clock in 1915, and the Allied armed forces followed soon after,[14]with the British Army switching officially in 1918.[15]The Canadian armed forces first started to use the 24-hour clock in late 1917.[16]In 1920, theUS Navywas the first US organization to adopt the system; theUS Army, however, did not officially adopt the 24-hour clock untilWorld War II, on July 1, 1942.[17]

In Britain, the use of the 24-hour clock in daily life has grown steadily since the beginning of the 20th century, although attempts to make the system official failed more than once.[18]In 1934, theBBCswitched 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.[18]In the same year, the US airlinesPan American World Airways CorporationandWestern Airlinesboth adopted the 24-hour clock.[19]In modern times, the BBC uses a mixture of both the 12-hour and the 24-hour clock.[18]

British RailandLondon Transportswitched to the 24-hour clock for timetables in 1964.[18]

TheShepherd Gate ClockwithRoman numbersup to

(23) and 0 for midnight, in Greenwich

Date and time representation by country

See theCommon Locale Data Repositoryfor detailed data about the preferred date and time notations used across the world, as well thelocalesettings of major computeroperating systems, and the articleDate and time notation by country.

International StandardISO 8601: Data elements and interchange formats Information interchange Representation of dates and ternational Organization for Standardization, 3rd ed., 2004.

Gloria D. Pickar (2011), Dosage Calculations, Cengage Learning, 537 pp. (p.60)[1]

U.S. Government Printing Office, Style Manual.

12. Numerals. Archived fromthe originalon 2008-04-21

Data elements and interchange formats Information interchange Representation of dates and times

2011-08-07 at theWayback Machine., Allied Communications Publication ACP 121(I), page 36, Combined Communications-Electronics Board, October 2010

SECNAV M-5216.5 Department of the Navy Correspondence Manual dated March 2010, Chapter 2, Section 5 Paragraph 15. Expressing Military Time.


History of the Hour. Clock and Modern Temporal Orders

. The University of Chicago Press. p.114.ISBN0226155110.

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

Memorandum CAB 24/110/21 (CP 1721), Report of the Committee upon the 24 hour method of expressing time

The National ArchivesKewUK. 4 August 1920.

Gallant Canadians: The Story of the 10th Canadian Infantry Battalion 19141919

Counting Time: a brief history of the 24-hour clock

Sarasota Herald Tribune 1943 May 14

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This page was last edited on 14 May 2018, at 10:29.

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