24 hour clock /ˌtwenti fɔːr aʊə ˈklɒk $ -aʊər ˈklɑːk/ BrE AmE 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.