# 24-hour clock
## English
### Etymology

From 24 +‎ hour +‎ clock.

### Pronunciation

* (Received Pronunciation) IPA: /ˌtwɛntiˈfɔːɹˌaʊə ˈklɒk/
* Audio (Southern England):(file)
* (General American) IPA: /ˌtwɛntiˈfɔɹˌaʊəɹ ˈklɔk/, [-ɾi-], (cot–caught merger) /-ˈklɑk/
* Rhymes: -ɒk

### Noun

24-hour clock (plural 24-hour clocks)
* A timekeeping convention in which the 24 hours of the day are treated as a single period rather than two sets of 12 hours, with the result that midnight is indicated as 00:00 (or sometimes 24:00), and the hours from 1:00 to 11:00 p.m. as 13:00 to 23:00.
    Synonyms: (chiefly UK) continental time, (chiefly US) military time
    Coordinate terms: 12-hour clock, twelve-hour clock
** 1893, “Time and Its Landmarks”, in J. C. Thomas, editor, Manual of Useful Information […] (The Working Teachers’ Library; V), Chicago, Ill.: The Werner Company, →OCLC, page 41:Twenty-four hour clock time is gaining in favor.
** 1899 February 23, Fred J. Miller, editor, American Machinist: A Practical Journal of Machine Construction, volume XXII, number 8, New York, N.Y.: American Machinist Press, →ISSN, →OCLC, page 33-157, column 3:The American Society of Civil Engineers recently announced a meeting of the society to occur at their house in New York at 20.30 o'clock, thus springing the twenty-four hour clock system upon those who wished to attend, and causing some inquiries and explanations as to the time intended. Commodore Melville, who was to be present, was one of those who had to inquire and to be informed that 8.30 P. M. was intended.
** 1909 November, James Arthur, “Time and Its Measurement: Part IV”, in H[enry] H[aven] Windsor, editor, Popular Mechanics, volume 12, number 5, Chicago, Ill.: Popular Mechanics Co., →ISSN, →OCLC, page 627, column 1:The sidereal clock of the astronomer does run to a regular motion; but our 24-hour clocks do not, as we shall see later.
** 1942, Harold E. Baughman, “Civil Air Communications: Reprinted from Manual of Operations: Chapter B—Communications Section”, in Baughman’s Aviation Dictionary and Reference Guide […], 2nd edition, Los Angeles, Calif.: Aero Publishers, →OCLC, paragraphs B5.1072–B5.10720, page 363:Time shall be stated in exactly four figures utilizing the twenty-four clock basis. […] Midnight is 0000, never 2400. The last hour of the twenty-four hour clock day begins at 2300. The last minute of the hour begins at 2359 and ends at 0000, which is the beginning of the first minute ending at 0001 of the first hour of the next day.
** 1961 December, “Talking of Trains: 24hr Clock Experiment at York”, in Trains Illustrated, London: Ian Allan Publishing, →ISSN, →OCLC, page 712:Train and steamer services to and from the Continent, including the boat trains in England, have long had 24hr clock indications, mainly to conform with the practice of connecting Continental railways, for Irish services are exempt.
** 1978, Lawrence D. Pedde, Warren E. Foote, LeRoy F. Scott, Danny L. King, Dave L. McGalliard, “Featured Units”, in Metric Manual, Denver, Colo.: [United States Government Printing Office for the] Bureau of Reclamation, U.S. Department of the Interior, →OCLC, page 43:Twenty-four-hour clock.—The use of the 24-hour clock eliminates the use of a.m. and p.m. Giving the time from midnight to 12.59 remains unchanged; the remaining customary afternoon times are increased by 12.
** 1989, Jane Briehl, “Thirty-eight Activity Cards [Minute Math Card #2: The Twenty-four Hour Clock!]”, in All about Time: Curriculum-based Activities for Telling Time & Much More!: Grades 4–6, [Napanee, Ont.]: S & S Learning Materials, →ISBN, page 23:The twenty-four hour clock is used by railroads, airports and the military because there is no need to add a.m. or p.m. The number on the clock immediately tells you if it is before or after noon. Eight thirty in the morning would be 08:30. Eight thirty at night would be 20:30.
** 2016, Caroline Potter, “Satie’s Texted Piano Works”, in Erik Satie: A Parisian Composer and His World, Woodbridge, Suffolk: The Boydell Press, Boydell & Brewer, →ISBN, page 130:Whiting [i.e., Stephen Moore Whiting, Satie the Bohemian (1999), page 410] remarks that '[Erik] Satie was evidently fascinated by the phenomenon of the government changing the designation of hours through simple imposition of the 24-hour clock. […] In 1952, René Chalupt recalled that Satie, "whenever he fixed the time of a meeting … would use both the new and old designation: 'I'll be there at 1800 hrs (6 o'clock old style)'".' […] The benefits and drawbacks of the twenty-four-hour clock were a topic of debate in early twentieth-century France. The guidebook of the Paris Exposition in 1900 states: 'Let's remember that Belgium is the only country to have officially adopted the 24-hour clock, just as the Observatory of Paris has done since 10 January 1900.'
** 2024, Bernard K. Means, “Ring-shaped Settlements and Exploratory Circular Statistics: A Graphical Approach”, in Martin Menz, Analise Hollingshead, Haley Messer, editors, The Archaeology of Arcuate Communities: Spatial Patterning and Settlement in the Eastern Woodlands (A Dan Josselyn Memorial Publication), Tuscaloosa, Ala.: University of Alabama Press, →ISBN, part II (Organizing Principles of Arcuate Communities), page 95:With circular or directional data, there is no true zero. The lowest value in a circular distribution is the same as its highest value. Common examples cited to illustrate this point are twenty-four hour clocks and twelve-month calendars.

#### Alternative forms

* twenty-four-hour clock

#### Related terms

* 24/7, twenty-four seven
* 24-hour day, twenty-four hour day
* 24-hour flu
* twenty-four hours a day

#### Translations
### Further reading

* 24-hour clock on Wikipedia.Wikipedia
* “twenty-four-hour, n.”, in OED Online ⁠, Oxford: Oxford University Press, September 2024.