day

Generally, a day is roughly the time of one rotation of the Earth (about 24 hours) or one rotation of other large astronomical objects. In everyday life, the word "day" often refers to a solar day, which is the length between two solar noons or times the Sun reaches the highest point. The word "day" may also refer to daytime, a time period when the location receives direct and indirect sunlight. On Earth, as a location passes through its day, it experiences morning, noon, afternoon, evening, and night. The effect of a day is vital to many life processes, which is called the circadian rhythm.
Days can be organized into calendars as dates, usually into months and years. Most calendars' arrangement of dates use either or both the Sun's (solar calendar) or the Moon's (lunar calendar) relative position in the sky. The start of a day is commonly accepted as roughly the time of the middle of the night or midnight, written as 0:00 or 12 am in 12- or 24-hour clock. Because the time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time.
In specific applications, the definition of a day are slightly modified, such as in the SI day (exactly 86,400 seconds) used for computers and standards keeping, local mean time accounting of the Earth's natural fluctuation of a solar day, and stellar day and sidereal day (using the celestial sphere) used for astronomy. In some countries, a day may be set starting earlier or later via the daylight saving time, making it possible to have a 23- or 25-hour civil day.

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