12-hour clock
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| 24-hour clock | 12-hour clock |
|---|---|
| 00:00 | 12:00 midnight |
| 01:00 | 1:00 a.m. |
| 02:00 | 2:00 a.m. |
| ... | ... |
| 11:00 | 11:00 a.m. |
| 12:00 | 12:00 noon |
| 13:00 | 1:00 p.m. |
| 14:00 | 2:00 p.m. |
| 15:00 | 3:00 p.m. |
| 16:00 | 4:00 p.m. |
| 17:00 | 5:00 p.m. |
| 18:00 | 6:00 p.m. |
| 19:00 | 7:00 p.m. |
| 20:00 | 8:00 p.m. |
| 21:00 | 9:00 p.m. |
| 22:00 | 10:00 p.m. |
| 23:00 | 11:00 p.m. |
| 24:00 | 12:00 midnight |
The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (a.m., Latin for "before noon") and post meridiem (p.m., Latin for "after noon"). Each period consists of 12 hours numbered 12, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. The a.m. period runs from midnight to noon, while the p.m. period runs from noon to midnight.
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History and usage
The 12-hour clock originated in EgyptTemplate:Citation needed. However, the lengths of their hours varied seasonally, always with 12 hours from dusk to dawn and 12 hours from dawn to dusk. The Romans also used a twelve-hour clock: the day was divided into twelve equal hours (of, thus, varying length throughout the year) and the night was divided into three watches. With solar time, before the advent of water clocks, there was no way to have a fixed hour.
The Romans numbered the morning hours originally in reverse: for example, "3 a.m.", or 3 hours ante meridiem meant "three hours before noon", compared to the modern meaning of "three hours after midnight".
Today, the 12-hour clock persists mostly in the form of analog clocks, where the smaller hand commonly rotates once every 12 hours (even in cultures where the 24-hour notation is otherwise preferred; a 24-hour hand advances only 15° per hour, which is too small an angle for easy reading.)
Although it has largely been replaced today by the 24-hour notation around the world, especially in written communication, the 12-hour notation with a.m./p.m. suffix remains today the dominant form of time written and spoken in Australia and the United States of America. It is also commonly used, alongside the 24-hour clock, in Canada (except Quebec), the United Kingdom, Albania, Greece, some other English-speaking regions, and some of the Spanish-speaking regions of South America. The Latin abbreviations "a.m." and "p.m." are used in English and Spanish. In Albanian, the equivalents are "PD" and "MD", and in Greek they are "πµ" and "µµ". Most other languages lack formal abbreviations for "before noon" and "after noon" and their people use the 12-hour clock only verbally and informally.
In Ethiopia, a 12-hour clock is still used that counts 12, 1, 2, ..., 10, 11 from dusk till dawn, and again 12, 1, 2, ..., 10, 11 from dawn to dusk. Unlike the convention in most countries, the start of the day is dawn, rather than midnight.
Criticism and practical problems
Many people who grew up with the 24-hour clock see the 12-hour notation as a less practical and outdated convention, especially in the context of written communication, computers and digital clocks. The arguments for or against a change to the more modern alternative are, in many ways, similar to the discussion on metrication.
In comparison to the 24-hour clock, the disadvantages most commonly voiced by critics of the 12-hour notation are:
- confusion about the correct notation for noon and midnight;
- confusion about the difference between midnight at the start and end of a given date;
- typographically, the a.m./p.m. suffix requires more space;
- the rollover from 12 a.m. to 1 a.m. happens one hour after the rollover from one day's date to the next;
- simple lexicographic string sorting in computer programs does not lead to the temporally correct sorting order;
- generally more complicated to understand and to implement in software and digital electronics.
Ambiguity at noon and midnight
The actual meaning of the terms ante meridiem (before noon) and post meridiem (after noon) are obviously not applicable at exactly noon or midnight.
However, it has become common practice in the United States to designate noon as 12:00 p.m and midnight as 12:00 a.m. The practical advantage of this convention becomes clear when one considers a digital clock. Noon and midnight are only infinitely short points in time, and therefore it is not practical to use any other convention than that which also applies immediately afterwards, when the clock still displays 12:00. This convention is standardized for computer usage in American National Standard ANSI INCITS 310 (which extends the international standard ISO 8601 time notation with a 12-h a.m./p.m. variant for the U.S.-market).
Many U.S. style guides (including the NIST website) recommend instead that it is clearest if one refers to "noon" or "12:00 noon" and "midnight" or "12:00 midnight" (rather than to 12:00 p.m. and 12:00 a.m., respectively). Some other style guides suggest "12:00 Template:Smallcaps" for noon and "12:00 Template:Smallcaps" for midnight, but this conflicts with the older tradition of using "12:00 Template:Smallcaps" for noon (Latin meridies), and "12:00 Template:Smallcaps" for midnight (media nox).
Even with all these conventions, references to midnight remain problematic, because they do not distinguish between the midnight at the start of the day referenced and the midnight at its end. Therefore, some U.S. style guides recommend to either provide other context clues, or to avoid references to midnight entirely, for example in favour of 11:59 p.m. for the end of the day and 00:01 a.m. for the start of the day. The latter has become common practice in the United States in legal contracts and for airplane, bus, or train schedules.
The 24-hour clock notation avoids all of these ambiguities by using 00:00, 12:00, and 24:00.
Related conventions
Typography
The initialisms "AM" and "PM" are variously written in small capitals ("Template:Smallcaps" and "Template:Smallcaps"), uppercase letters ("AM" and "PM"), or lowercase letters ("am" and "pm"). Additionally, some styles use periods (full stops), especially in combination with lowercase letters (thus "a.m." and "p.m.").
Some style guides suggest not to use a leading zero with a single-digit hour; for example, "3:52 p.m." is preferred over "03:52 p.m.". Many digital clocks nevertheless use a leading zero. (The leading zero is more commonly used with the 24-hour notation; especially in computer applications it can help to maintain column alignment in tables and correct sorting order.)
There are symbols for "AM" (㏂ = "㏂") and "PM" (㏘ = "㏘") in Unicode. However, they are meant to be used only with CJK fonts, as they take up exactly the same space as one Chinese character.
Pronunciation
Times of day ending in :00 minutes may be pronounced in English as the numbered hour followed by o'clock (e.g., 10:00 ten o'clock, 2:00 two o'clock, 4:00 four o'clock etc). This may be followed by the a.m. or p.m. designator, or may not be if obvious. O'clock itself may be omitted, leaving a time like four a.m. or four p.m.. :01 through :09 are usually pronounced as oh one through oh nine (though ought one through ought nine may still be in use in some Commonwealth countries). :10 through :59 are their usual number-words.