Midnight - Start and End of Day

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, authorities recommend avoiding confusion between noon and midnight by using "midnight", "12 midnight", or "12:00 midnight."

In America, 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 changes to 12:00. Simultaneously, the p.m. changes 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, "0:00" and "0:00:00" refer to midnight at the start of a given date. Some styles, such as ISO 8601, allow 24:00 to refer to the end of a day. Noon is 12:00:00.

While computers and digital clocks display "00:00 a.m." and "00:00 p.m.", those notations provide no clear and unambiguous way to distinguish between midnight and noon. It is actually grammatically incorrect 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 is neither after noon nor before noon, and midnight can equally be twelve hours before and after 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).

The most common ways to represent these times are, (a) to use a 24-hour clock (0:00 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 the time 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.," as is used where the ambiguity can have serious consequences, such as in contracts and insurance policies.

The thirtieth edition of the U.S. Government Style Manual (2008) sections 9.54 and 12.9b recommends the use of "12 a.m." for midnight and "12 p.m." for noon. Some religious calendars continue to begin the day at another time — for example, at dusk in the Hebrew calendar and the Islamic calendar.