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The 24-hour clock is the convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today,^[1] and is the international standard (ISO 8601) notation for time of day.^[2] In the practice of medicine, the 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patient's medical history.^[3] It is popularly referred to as military time or astronomical time in the United States, Canada,^[4] and a handful of other countries^[1] where the 12-hour clock is still dominant.

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http://en.wikipedia.org/wiki/24-hour_clock