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The American Heritage Dictionary of the English Language states "By convention, 12 AM denotes midnight and 12 PM denotes noon. Because of the potential for confusion, it is advisable to use 12 noon and 12 midnight". [34] E. G. Richards in his book Mapping Time (1999) provided a diagram in which 12 a.m. means noon and 12 p.m. means midnight. [35]
Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 midday), 12 p.m. (for post meridiem, literally "after midday"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 (military time). Solar noon is the time when the Sun appears to contact the local celestial meridian.
Every digital clock that uses am/pm assumes that 12:00 am is midnight and 12:00 pm is noon. It is the de facto convention, and rightly so. I have seen increasing use of 12:00 pm in signs and notices and such to mean noon and never the other way around. The American Heritage dictionary says 12:00 am is midnight and 12:00 pm is noon.
Strictly speaking, it is incorrect to use "a.m." and "p.m." when referring to noon or midnight. 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 before nor after noon, and midnight is exactly twelve hours before and after noon, neither abbreviation is correct.
The English term noon is also derived from the ninth hour. This was a period of prayer initially held at three in the afternoon but eventually moved back to midday for unknown reasons. [12] The change of meaning was complete by around 1300. [13] The terms a.m. and p.m. are still used in the 12-hour clock, as opposed to the 24-hour clock.
12 (noon) is normally 12 PM for the whole afternoon hour between 12:00 and 12:59. 13..23 (afternoon and start of night) is unambiguously 01..11 PM. 24 (midnight at end of day) is normally 12 AM (but the date must be adjusted when using the 12-hour format).
It appears that in the original 12 hour time "system" the anti meridiem and post meridiem refer to hours past the hour of noon (the meridiem), therefore 12 noon was neither am nor pm and midnight was both 12 hours anti meridiem and 12 hours post meridiem. Perhaps this is why there is so much confusion.
German (Gallic) hours (half-dial): 2×12 hour system starting at midnight and restarted at noon. It is typical with the 12-hour dial with 12 at the top. The modern 24-hour system is a late-19th century adaptation of the German midnight-starting system, and then prevailed in the world with the exception of some Anglophone countries.