12-hour clock

24-hour clock	12-hour clock
00:00	12:00 midnight (a.m.)
01:00	1:00 a.m.
02:00	2:00 a.m.
...	...
11:00	11:00 a.m.
12:00	12:00 noon (p.m.)
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 (a.m.)

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., from Latin, literally "before midday") and post meridiem (p.m., "after midday"). Each period consists of 12 hours numbered 12 (acting as a zero), 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. The most common convention is to assign 12 a.m. to midnight and 12 p.m. to noon, defining both half days to have a closed (inclusive) beginning and open (exclusive) end.

History and usage

The 12-hour clock originated in Egypt. However, the lengths of their hours varied seasonally, always with 12 hours from sunrise to sunset and 12 hours from sunset to sunrise, the hour beginning and ending each half-day (four hours each day) being a twilight hour. An Egyptian sundial for daylight useBerlin instruments of the old Egyptian time of day destination and an Egyptian water clock for nighttime use found in the tomb of Pharaoh Amenhotep I,A Walk through Time - Water Clocks both dating to c. 1500 BC, divided these periods into twelve hours each. 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.

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".

Early medieval clock faces used the 12 hour numbering scheme (using Roman numerals), but showed both a.m. and p.m. sequences simultaneously. This is known as the Double-XII system, and can be seen on surviving medieval clock faces, such as those at Ottery St Mary, Wells, and Venice.

Today, most analog clocks use the 12-hour clock dial, where the hour hand (shorter and sometimes thicker) commonly rotates once every 12 hours. These are used even in cultures where the 24-hour notation is otherwise preferred. Some 12-hour dials show the numbers 13 to 23 written inside the primary 1 to 12 ring.

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 is common in some parts of the world.

It is the dominant form of time written and spoken in:

It is commonly used alongside the 24-hour clock in:

Canada (except Quebec)
Albania
Bulgaria
Greece
United Kingdom and other English-speaking regions
some Spanish-speaking regions of South America

Use by country

Main article: date and time notation by country

United States

The United States differs from other countries in that it uses the 12-hour clock almost exclusively, and has no plans to change. The US military uses the 24-hour clock, and so it is commonly called military time in the US.

United Kingdom

Both the 12-hour and 24-hour notations are used in the United Kingdom. The 12-hour notation is still widely used in ordinary life, written communication and displays and continues to be used in informal spoken language. The 24-hour notation is used in timetables and some written communication, but its use there is not universal as in much of the non-English speaking world.

Canada

In Canada, similar to the United States and the United Kingdom, the 12-hour clock is used in ordinary life. An exception to this, however, is in Quebec, where French speakers mostly use the 24-hour clock. 24-hour time is also very common in the capital, Ottawa, Ontario.

Non-English speaking world

The 24-hour clock enjoys broad everyday usage in most Asian, European and many Latin American countries . When a time is written or displayed, the 24-hour notation is used in these countries almost exclusively . The 12-hour clock remains in some regions commonly used in informal language, while, for example, most German, French and Romanian speakers use the 24-hour clock today even when speaking casually.

It is not uncommon that the same person would use the 24-hour notation in spoken language when referring to an exact point in time ("The train leaves at fourteen forty-five …"), while using some variant of the 12-hour notation to refer vaguely to a time ("… so I will be back tonight sometime after five."). People are used to converting between the two notations without requiring mental arithmetic, and most perceive "three o'clock" and "15:00" simply as synonyms.

In many languages, an equivalent of a.m./p.m. is not normally used, but instead the numbers of the 12-hour clock are used with an added phrase like "in the night/morning/afternoon/evening", reflecting an underlying 6-hour clock.

Ethiopia

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.

Thailand

In Thailand, next to the 12-hour and 24-hour clock system, also a 6-hour clock system is use, especially in spoken language. It counts 4 times from 1 to 6, with different additional words to make the distinction for night, morning, afternoon and evening.

Abbreviations

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 users use the 12-hour clock only verbally and informally.

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;
the lexicographical order does not equal the chronological order;
generally more complicated to understand and to implement in software and digital electronics.
the day starts at 12 a.m. instead of 6 a.m. when the sun comes up.

Ambiguity at noon and midnight

The literal meaning of the terms ante meridiem (before noon) and post meridiem (after noon) are not applicable at exactly noon or midnight.

However, it has become common practice in countries that use the system (such as 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 infinitesimal 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 " for noon and "12:00 " for midnight, but this conflicts with the older tradition of using "12:00 " for noon (Latin meridies), and "12:00 " 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 12:01 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.

Advantages over the 24-hour clock

AM-PM can sometimes take up less space.

Virtually all analog clocks and watches have dials of 12 hours. Every hour position has a single name, with just a suffix to separate the two versions, while a 24-hour clock assigns two names to each position or requires some mental arithmetic.

Clocks with a chime are in step only on hours 1 to 12, and deviate for hours 0, and 13 to 23. Chiming 0 times is not a possible signal and chiming up to 24 times becomes annoying and difficult to count.

Some people find the idea of 0 or 00 hours unnatural (even though they may be perfectly happy with 00 minutes because they are place holders). In the 12-hour clock 12 takes the place of 0, therefore the day rolls over at 11:59-12:00 not 12:59-1:00.

Restricting the hour number range to 1–12 can accommodate a preference for monosyllabic words, which may be particularly associated with the English language.

Some people claim that changing the notation of the clock to 24-hours destroys cultural values and identity and abominates the language.

Some people dislike the idea that midnight has two different notations in the 24-hour clock, depending on whether it is the beginning (0) or end (24) of the day.

Related conventions

Typography

The initialisms "AM" and "PM" are variously written in small capitals ("" and ""), 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.

Military circles use the 24-hour clock exclusively and would typically pronounce times ending in :00 minutes as the hour followed by "hundred" with a optional "hours". For instance, 16:00 would be pronounced "sixteen hundred" or "sixteen hundred hours".

References

External links

Clocks | Timekeeping

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