A.M. and P.M.

Non-abbreviated form: Ante Meridiem and Post Meridiem

Is noon AM or PM?

Correct use of AM and PM

Noon is 12 PM (12:00 hrs). Midnight is 12 AM (00:00 hrs).

AM begins immediately after midnight, and PM begins immediately after noon.

The universal convention is that 12:00 AM signifies midnight, and every moment afterwards is also AM until the precise moment of 12:00 PM, which is midday.

Therefore every day starts precisely at midnight, and AM starts immediately so that 00:00:00 (24-hour format) is the first moment of AM (see also leap seconds).

When scheduling around midnight, some use 23:59 to mean "same-day" and 00:01 to mean "next day." But it is best to use the correct date, since 00:00 is always same-day; there is no option within the 24-hour notation for specifying tomorrow.

More on Time of Day

Under consideration for clock time is a notional Prime Meridian (or meridien for the more archaic Anglo-French spelling); this is a line of longitude that is measured rotating in view of the Sun and makes practical world time possible. At midday, 12:00 PM GMT, the sun is directly above this notional line in Greenwich, London, and at midnight the sun is directly opposite on the other side of the Earth. The interchange of the terms Meridiem and Meridian is interesting, since the former refers to time and the latter to space.

The discovery of relativity in the early 20th century led to the understanding of space and time as a combined dimension. Einstein referred to this dimension as "space-time." You cannot measure time without a spatial marker and constant velocity, and you cannot measure velocity without assessment of both space over a measured interval, leading to impossible, circular definitions.

Time is therefore a metaphysical quantity that we may only approach indirectly through spatial observation and cumulative effect. Over a period of time there is relentless energy dissipation, and increasing disorder on the one hand, versus entropy-defying growth (life), which is an accumulating order, on the other.

Life, of course, is another metaphysical quantity for which there is no adequate scientific definition.

What do the initials AM and PM mean?