24-hour clock

plural 24-hour clocks · alternative twenty-four hour clock · alternative twenty-four-hour clock

1. 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.

   Twenty-four hour clock time is gaining in favor.

   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

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   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.

   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 sidereal clock of the astronomer does run to a regular motion; but our 24-hour clocks do not, as we shall see later.

   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

   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.

   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

   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.

   1961 December, “Talking of Trains: 24hr Clock Experiment at York”, in Trains Illustrated, London: Ian Allan Publishing, →ISSN, →OCLC, page 712

   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.

   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

   Synonyms: continental time, military time