Days

In common usage, a day starts at midnight, written as 00:00 or 12:00 am in 24- or 12-hour clocks, respectively. Because the time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time. Other conventions exist for marking the beginning and end of days, such as the Jewish religious calendar which counts days beginning at sunset, or in astronomy, in which a day begins at noon so that observations throughout a single night are recorded as happening on the same day.