ISO 8601
iso-8601
An international standard for representing dates and times in an unambiguous, machine-readable format.
24-Hour Clock
24-hour clock
Overview
The 24-hour clock represents the time of day using numbers from 0:00 (midnight) to 23:59 (one minute before the next midnight). Unlike the 12-hour (AM/PM) system, it requires no prefix to distinguish morning from afternoon, removing all ambiguity. ISO 8601 mandates 24-hour notation, making it the international standard for data exchange.
Regional Usage
Continental Europe uses 24-hour notation throughout daily life. In Japan, South Korea, and China everyday speech favors 12-hour expressions, while timetables, broadcast listings, and official notation follow the 24-hour clock. In the United States, Canada, and Australia the 12-hour format also dominates casual conversation. The military, however, has adopted the 24-hour clock officially, and in the United States it is known as "military time". Medical records and aviation likewise treat 24-hour notation as the standard because it prevents misreadings. The United Kingdom sits in between, using 12-hour speech informally while adopting 24-hour format in timetables and official documents.
Programming Considerations
Internally, software stores hours as integers 0 through 23 and converts to 12-hour display only when presenting to users whose locale expects it. JavaScript's Intl.DateTimeFormat, for example, provides an hour12 option that toggles the display format. Databases and log files should always record times in 24-hour notation to avoid parsing errors and ambiguity.
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Related Terms
Time Zone
timezone
A region of the globe that observes a uniform standard time for legal, commercial, and social purposes.
Zulu Time
zulu time
Zulu time is the military and aviation term for UTC, derived from the NATO phonetic alphabet designation Z (Zulu) for the zero-meridian time zone. The trailing Z in ISO 8601 timestamps is the zone designator for the zero UTC offset, and because it is the same letter as the military Zulu zone, it is also called Zulu time.
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