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.
But it is unknown as to whether "midnight" belongs to the previous day or the next day. I.e., if we are to meet at "midnight December 24th," is that at the beginning of Dec. 24th or the end?
With
12-hour time notation, most authorities recommend avoiding confusion by using "midnight", "12 midnight", or "12:00 midnight".[
citation needed]
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, 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 "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.
[1]
The 30th 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.
[2][3][nb 1]