24 hour clock /ˌtwenti fɔːr aʊə ˈklɒk $ -aʊər ˈklɑːk/ noun
a clock, or a system of telling the time, according to which, for example, one o'clock in the afternoon is called 1300 hours (pronounced ‘13 hundred hours’):
The trains run according to the 24 hour clock.

[TahlilGaran] Dictionary of Contemporary English ▲