Midnight is, literally, "the middle of the night." In most systems it is when one day ends and the next begins: when the date changes. Originally midnight was halfway between sunset and dawn, varying according to the seasons.

Solar midnight is that time opposite of solar noon, when the sun is closest to nadir and the night is equidistant from dusk and dawn. Due to the advent of time zones, which make time identical across a range of meridians, and daylight saving time, it rarely coincides with midnight on a clock, but can be computed on a number of websites that perform solar time calculations. Solar midnight is dependent on longitude, latitude, altitude, and time of the year rather than on a time zone.

Start and end of day

Midnight marks the beginning and ending of each day in civil time throughout the world. It is the dividing point between one day and another.

With 12-hour time notation, most authorities recommend avoiding confusion by using "midnight", "12 midnight", or "12:00 midnight".

Digital clocks and computers commonly display 12 a.m. for midnight. While that phrase may be used practically, it helps to understand that any particular time is actually an instant. The "a.m." shown on clock displays refers to the 12-hour period following the instant of midnight, not to the instant itself. In other words, 11:59 p.m. shows until midnight; at the instant of midnight it flips to 12:00. Simultaneously, the p.m. flips to a.m., though, strictly speaking, a.m. does not apply to the instant of midnight which separates p.m. and a.m.

In 24-hour time notation, "00:00" and "00:00:00" refer to midnight at the start of a given date. Some styles allow 24:00 for the end of a day. Noon is 12:00:00.

While computers and digital clocks display "12:00 a.m." and "12:00 p.m.", those notations provide no clear and unambiguous way to distinguish between midnight and noon. It is actually improper to use "a.m." and "p.m." when referring to 12:00. The abbreviation a.m. stands for ante meridiem or before noon and p.m. stands for post meridiem or after noon. Since noon and midnight are neither after noon nor before noon, neither abbreviation is correct (although the length of the error is determined by the smallest unit of time — 12:00:01 p.m. would be correctly notated). Similarly, midnight is both twelve hours before as well as twelve hours after noon, so both are ambiguous as to the date intended.

The most common ways to represent these times are, (a) to use a 24-hour clock (00:00 and 12:00, 24:00), (b) to use "12 noon" or "12 midnight", although unless the person is referring to a general time and not a specific day, "12 midnight" is still ambiguous, (c) to specify it between two successive days or dates (Midnight Saturday/Sunday or Midnight December 14/15), and (d) to use "12:01 a.m." or "11:59 p.m." This final usage is common in the travel industry, especially train and plane schedules, to avoid confusion as to passengers' schedules. The U.S. Government Printing Office Style Manual recommends the opposite, that noon is 12 a.m. and midnight is 12 p.m.