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Is noon 12 PM or 12 AM?
My peeve is with people who assume others know which rule they are using when they say 12 pm or 12 am. Sometimes it's obvious from the context whether they mean noon or midnight, but sometimes not.
First, let's have a look at what some official guvmint agencies have to say.
In the United States, the Time and Frequency Division of the NIST (an agency of the Department of Commerce) addresses this issue on their FAQ webpage:
This is a tricky question. The answer is that the terms 12 a.m. and 12 p.m. are wrong and should not be used.
To illustrate this, consider that "a.m" and "p.m." are abbreviations for "ante meridiem" and "post meridiem." They mean "before noon" and "after noon," respectively. Noon is neither before or after noon; it is simply noon. Therefore, neither the "a.m." nor "p.m." designation is correct. On the other hand, midnight is both 12 hours before noon and 12 hours after noon. Therefore, either 12 a.m. or 12 p.m. could work as a designation for midnight, but both would be ambiguous as to the date intended.
When a specific date is important, and when we can use a 24-hour clock, we prefer to designate that moment not as 1200 midnight, but rather as 0000 if we are referring to the beginning of a given day (or date), or 2400 if we are designating the end of a given day (or date).
To be certain of avoiding ambiguity (while still using a 12-hour clock), specify an event as beginning at 1201 a.m. or ending at 1159 p.m., for example; this method is used by the railroads and airlines for schedules, and is often found on legal papers such as contracts and insurance policies.
If one is referring not to a specific date, but rather to several days, or days in general, use the terms noon and midnight instead of 12 a.m. and 12 p.m. For example, a bank might be open on Saturdays from 8 a.m. to noon. Or a grocery store might be open daily until midnight. The terms "12 noon" and "12 midnight" are also correct, though redundant.
Copyright © National Institute of Standards and Technology
And according to the FAQ at the British Royal Observatory Greenwich:
12 noon is neither a.m. nor p.m.
To avoid confusion, the correct designation for twelve o'clock is 12 noon or 12 midnight. Alternatively, the twenty-four-hour-clock system may be used.
The abbreviation a.m. stands for ante meridiem (before the Sun has crossed the line) and p.m. for post meridiem (after the Sun has crossed the line). At 12 noon, the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither "ante-" nor "post-".
Copyright © National Maritime Museum.
The Greenwich2000® website says something similar.
[see also http://greenwichmeantime.com/info/noon.htm]
These explanations are of little or no help with setting one's alarm clock or VCR for noon or midnight. Such devices usually only offer the option of 12 PM or 12 AM, in the North America, anyway.
Dictionaries and Encyclopedias
Next, let's look at what some dictionaries and encyclopedias have to say about it.
Eric Weisstein's World of Astronomy defines post meridiem:
Being after noon. "Post meridiem" is commonly abbreviated p.m., P.M., or PM, and is used in time designations in the United States 12-hour clock system to indicate the afternoon hours. Noon itself is neither a.m. nor p.m., but the minute between 12:00 and 12:01 is p.m., so 12:00 noon is often denoted p.m. on digital clocks in the U. S.
The American Heritage® Dictionary of the English Language, Fourth Edition, has this usage note regarding AM and PM:
By definition, 12 a.m. denotes midnight, and 12 p.m. denotes noon, but there is sufficient confusion over the meanings of a.m. and p.m. when the hour is 12 to make it advisable to use 12 noon and 12 midnight where clarity is required.
Copyright © 2000 by Houghton Mifflin Company
By definition? Where did this definition come from? Certainly not from any official guvmint agency quoted above.
Chicago Manual of Style
I could find no writing style guide or manual online that defines 12 PM as noon, but rather they advise against using that designation. Curiously, the Chicago Manual of Style (which is not online) has an example on page xxx that uses 12 PM for midnight.
The Chicgao Manual of Style, 14th Edition uses the example 12 PM (midnight) http://www.press.uchicago.edu/Misc/Chicago/cmosfaq.html
ISO 8601
A list of links about ISO 8601 can be found in the Open Directory Project:
http://dmoz.org/Science/Reference/Standards/Individual_Standards/ISO_8601/
A FAQ about ISO 8601:
http://dmoz.org/Science/Reference/.../ISO_8601/faq.html
The 2000-12-15 final draft of ISO 8601 can be downloaded from:
http://www.pvv.org/~nsaa/ISO8601.html
OK, so I'm a moron for putting up this page long before it's ready for prime time.
But I have a great vision that one day it will actually be finished and complete. Please share in this delusion.
[the following miscellaneous notes are not properly organized or formatted]
From: http://sciastro.astronomy.net/sci.astro.3.FAQ
Subject: C.02 What are all those different kinds of time? Author: Paul Schlyter <pausch@saaf.se>, Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>, Paul Eggert <eggert@twinsun.com>
APPARENT (or TRUE) SOLAR TIME: Still later, the hours were made equally long: the day+night was 24 hours. The "day" now started at midnight, not at sunset, which was marked as 00:00 (or 12:00 midnight in English time format). Noon was at 12:00 (or 12:00 noon in English time format). This is what we now refer to as "true solar time"---it is the time shown by a properly setup sundial. This time is local, it is different for different longitudes. (In strict English construction, 12:00 cannot be given either an A.M. = ante meridiem or P.M. = post meridiem designation, but it has become common to use 12 A.M. to mean midnight and 12 P.M. to mean noon. In traditional English, 12 M. = meridies means _noon_; nowadays one is just as likely to see 12 M. = midnight and 12 N. = noon.)
(In general, the old English A.M./P.M. notation is extremely problematic. A shorter and more obvious time notation is the modern 24h notation in which the hours in the day range from 00:00 to 23:59. This notation even allows one to distinguish midnight at the start of the day [00:00] from midnight at the end of the day [24:00], while the old English notation requires kludges like starting a contract at 12:01 A.M. in order to make clear which of the two midnights associated with a date had been intended. The 24h notation is the official international standard time notation (ISO 8601) and displayed by almost all digital clocks outside the U.S.A. The 24h notation is also recommended by the U.S. Naval Observatory in Washington, which defines official time in the U.S.)
http://www.cl.cam.ac.uk/~mgk25/iso-time.html
by Markus Kuhn's commentary about International Standard ISO 8601 specifies numeric representations of date and time.
Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
A remark for readers from the U.S.:
The 24h time notation specified here has already been the de-facto standard all over the world in written language for decades. The only exception are a few English speaking countries, where still notations with hours between 1 and 12 and additions like "a.m." and "p.m." are in wide use. The common 24h international standard notation is widely used now even in England (e.g. at airports, cinemas, bus/train timetables, etc.). Most other languages don't even have abbreviations like "a.m." and "p.m." and the 12h notation is certainly hardly ever used on Continental Europe to write or display a time. Even in the U.S., the military and computer programmers have been using the 24h notation for a long time.
The old English 12h notation has many disadvantages like:
It is longer than the normal 24h notation.
It takes somewhat more time for humans to compare two times in 12h notation.
It is not clear, how 00:00, 12:00 and 24:00 are represented. Even encyclopedias and style manuals contain contradicting descriptions and a common quick fix seems to be to avoid "12:00 a.m./p.m." altogether and write "noon", "midnight", or "12:01 a.m./p.m." instead, although the word "midnight" still does not distinguish between 00:00 and 24:00 (midnight at the start or end of a given day).
It makes people often believe that the next day starts at the overflow from "12:59 a.m." to "1:00 a.m.", which is a common problem not only when people try to program the timer of VCRs shortly after midnight.
It is not easily comparable with a string compare operation.
It is not immediately clear for the unaware, whether the time between "12:00 a.m./p.m." and "1:00 a.m./p.m." starts at 00:00 or at 12:00, i.e. the English 12h notation is more difficult to understand.
Please consider the 12h time to be a relic from the dark ages when Roman numerals were used, the number zero had not yet been invented and analog clocks were the only known form of displaying a time. Please avoid using it today, especially in technical applications! Even in the U.S., the widely respected Chicago Manual of Style now recommends using the international standard time notation in publications.
http://www.latviansonline.com/about/style.shtml
Particularly in calendar listings, but also in all articles, references to time are to be presented in 12-hour format with a.m. and p.m. denoting morning and afternoon or evening, respectively. Proper use: 3 a.m., 5:03 a.m., 7:30 p.m. The midday hour is noon, not 12 noon. Midnight is midnight, not 12 midnight.
