12H

12h refers to a span of twelve hours, or to the 12-hour timekeeping system commonly used to express daily time in many regions. In the timekeeping sense, the day is divided into two cycles of twelve hours each, producing a repeating 12-hour cycle. Times are typically marked with a.m. or p.m. to distinguish morning from afternoon and evening, for example 9:00 a.m. or 4:30 p.m. A 12-hour clock is often paired with an analog display that cycles through the same twelve-hour sequence twice in a 24-hour day.

Historically, divisions of the day into twelve hours trace to ancient civilizations and the use of base-12 counting. Mechanical clocks in medieval Europe popularized the two 12-hour cycles, and the abbreviations a.m. and p.m. emerged to reduce ambiguity in daily life. In modern practice, the 12-hour system remains common in conversational speech and many informal contexts, particularly in the United States and other parts of the world, while many formal timetables and systems prefer the 24-hour clock to avoid confusion.

In notation and data contexts, 12h can also denote a duration of twelve hours. In ISO 8601 duration notation, for example, a twelve-hour interval is written as PT12H. In practice, the same 12h designation may appear in schedules, logs, or documentation to indicate a time interval or shift length corresponding to twelve hours.

See also: 24-hour clock, AM/PM notation, time notation, ISO 8601 durations.