Noon - Nomenclature

Nomenclature

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

Digital clocks and computers commonly display 12 p.m. for noon. While that phrase may be used practically, it helps to understand that any particular time is actually an instant. The "p.m." shown on clock displays refers to the 12-hour period following the instant of noon, not to the instant itself.

While computers and digital clocks display "12:00 a.m." and "12:00 p.m." these notations provide no clear and unambiguous way to distinguish between midnight and noon. It is technically improper to use "a.m." and "p.m." when referring to 12:00. The abbreviation a.m. stands for ante meridiem (or before the meridian) and p.m. stands for post meridiem (or after the meridian), with the meridian being 12:00 noon. For this reason, neither abbreviation is correct for noon or midnight. The length of the error is determined by the smallest unit of time: 12:00:01 p.m. would be correctly notated, as would even 12:00:00.00001 pm.

The most common ways to represent these times are:

• to use a 24-hour clock (00:00 and 12:00, 24:00; but never 24:01)
• 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
• to specify midnight as between two successive days or dates (Midnight Saturday/Sunday or Midnight December 14/15)
• 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 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.