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twenty-four-hour clock

American  
[twen-tee-fawr-ouuhr klok, ou-er, twuhn-tee-] / ˈtwɛn tiˈfɔrˌaʊər ˈklɒk, ˌaʊ ər, ˈtwʌn ti- /
Or 24-hour clock

noun

  1. a system of time measurement in which hours in the day are counted off consecutively from 0 to 24, with midnight being either 00:00 or 24:00.

Etymology

Origin of twenty-four-hour clock

First recorded in 1780–90

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.