24-hour clock
A timekeeping system that numbers each hour of the day from 00 through 23. It distinguishes times without using a.m. or p.m.
Midnight: The instant when one calendar day ends and the next begins, conventionally 00:00. The clock’s count begins at 00:00 at this boundary between days.
Egyptian calendar: The civil calendar of ancient Egypt, organized into 365 days and three seasons. Ancient Egyptian timekeeping divided daylight and night into hours, an early precedent for numbered daily hours.
Military time: A time notation using 24-hour numbers, often spoken with conventions such as “fourteen hundred hours.” Military organizations use 24-hour notation to prevent ambiguity in orders and schedules.
12-hour clock: A timekeeping system that numbers hours from 1 to 12 twice each day, distinguishing periods with a.m. and p.m. It represents the same day with repeated hour numbers and requires a.m. or p.m. to disambiguate them.
Noon: The middle of the day, conventionally 12:00 in civil time. At 12:00, the hour numbers pass from morning hours to afternoon hours.
Babylonian astronomy: The mathematical study of celestial motions developed in ancient Babylonia. Babylonian sexagesimal arithmetic helped shape the hour and minute divisions used in modern timekeeping.
Railway timetable: A published schedule listing train departure, arrival, and connection times. Rail timetables commonly use 24-hour times to distinguish services throughout the day.
UTC: Coordinated Universal Time, the primary global standard for civil timekeeping. UTC is a time standard, not a clock notation, though it is commonly written with 24-hour hours.
Hour: A unit of time equal to 60 minutes or 3,600 seconds. Each numbered position on the clock marks one hour.
Mechanical clock: A clock that measures time through the regulated motion of mechanical components. Mechanical clocks made continuous, numbered hours practical in public and domestic life.