12-hour clock

24-hour clock	12-hour clock
00:00	midnight
01:00	1:00 a.m.
02:00	2:00 a.m.
...	...
11:00	11:00 a.m.
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 the same day	(See below)

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 the middle of the day", idiomatically "approaching midday") and post meridiem (p.m., "past midday"). Each period consists of 12 hours numbered 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Time periods run 12:01 through 11:59 either ante meridiem or post meridiem. When referencing twelve o'clock the phrase is either noon or midnight, without the ambiguity.

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 use and an Egyptian water clock for nighttime use found in the tomb of Pharaoh Amenhotep I, both dating to c. 1500 BC, divided these periods into 12 hours each.

The Romans also used a 12-hour clock: the day was divided into 12 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".

The first mechanical clocks in the 14th century, if they had dials at all, showed all 24 hours, using the 24 hour analog dial, influenced by astronomers' familiarity with the astrolabe and sundial, and their desire to model the apparent motion of the sun. In Northern Europe, these dials generally used the 12 hour numbering scheme in Roman numerals, but showed both a.m. and p.m. periods in sequence. This is known as the Double-XII system, and can be seen on many surviving clock faces, such as those at Wells and Exeter. Elsewhere in Europe, particularly in Italy, numbering was more likely to be based on the 24 hour system (I to XXIV), reflecting the Italian style of counting the hours.

Exeter Cathedral clock, showing the Double-XII numbering scheme.

During the 15th and 16th centuries, the 12 hour analog dial and time system, with its simpler and more economical construction, gradually became established as standard throughout Northern Europe for general public use, with the 24 hour analog dial reserved for the more specialist applications, such as for astronomical clocks and chronometers.

Today, most analog clocks and watches use the 12-hour dial, where the hour hand (shorter and sometimes thicker) commonly rotates once over the course of every 12 hours, and twice in a day. 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.

Use by country

Main article: Date and time notation by country

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. and p.m. suffixes is common in some parts of the world.

• The 12-hour clock is the dominant system of time written and spoken in:

• Australia
• Canada (except Quebec)
• New Zealand
• Philippines
• United States of America

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

• Albania
• Bulgaria
• Greece
• Quebec, Canada
• Turkey
• United Kingdom and other English-speaking regions
• some Spanish-speaking regions of South America

In many European countries, a 12-hour clock is commonly used in informal speech, but a.m. and p.m. are little known. If one wants to unambiguously refer to time in the 12-hour system, one uses descriptive phrases instead, such as in the morning, in the afternoon, in the evening, at night.

Abbreviations

The Latin abbreviations "a.m." and "p.m." (often written "am" and "pm"; "AM" and "PM"; and "A.M." and "P.M.") are used in English and Spanish. The equivalents in Albanian are "PD" and "MD", in Greek they are "πµ" and "µµ", and in Swedish (though in Sweden the 12-hour clock is nowadays rarely used) they are "f.m." and "e.m.". 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. Even most people who grew up with the 12-hour clock, however, have problems indicating midnight and noon. Those who grew up with the 24-hour clock are also confused when they come across situations very common in, for example, Internet forums and email in which a message indicated as posted at "12:46 am" appears unintuitively and seemingly illogically before a message marked "11:05 am" for example.

The disadvantages most commonly voiced by critics in comparing the 12-hour notation to the 24-hour clock 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. and p.m. suffixes require more space;
• the rollover from 12 midnight 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;
• it's generally more complicated to understand and to implement in software and digital electronics.

Confusion at noon and midnight

Style	Midnight (begin of day)	Noon	Midnight (end of day)
24-hour clock	00:00	12:00	24:00
de facto	12:00 a.m.	12:00 p.m.	—
antiquated	12:00 m.n.	12:00 m.	—
U.S. Government Printing Office Style Manual	—	12 a.m.	12 p.m.
NIST1	Midnight	Noon	Midnight
NIST2	Midnight	Noon	Midnight
U.S. de facto legal	12.01 a.m.	—	11.59 p.m
Handbook of Chemistry and Physics seventh edition 1941	—	12:00 a.m. better 12:00 M	12:00 p.m.

The terms ante meridiem (before noon) and post meridiem (after noon) do not literally apply to noon and midnight. Although the term meridies (literally midday) properly applies to noon, its abbreviation m. is not commonly used.

However, it has become common practice in countries that use the system (such as the United States) to designate 12:00 p.m. as noon and 12:00 a.m. as midnight at the beginning of the day (this is the de facto convention). 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 an ANSI standard (which extends the international standard ISO 8601 time notation with a 12-hour a.m./p.m. variant for the U.S.-market). [An easy way to remember this convention is to consider what the a.m./p.m. value would be one minute later at 12:01. Whatever it is at 12:01, that's what it is one minute earlier at 12:00. For example, suppose you find a file on your computer dated 12:00 p.m. and you wonder whether that means noon or midnight. Consider 12:01 p.m. That would be one minute after noon. Therefore, 12:00 p.m. denotes noon.]

Nevertheless, this usage of using 12:00 a.m. for midnight and 12:00 p.m. for noon is contrary to the U.S. Government Printing Office Style Manual which recommends the use of 12 p.m. for midnight and 12 a.m. formerly 12 m. for noon.

On the other hand, some references agree with the de facto convention. For example, in the American Heritage Dictionary (Second College Edition) the usage note in the entry for ante meridiem says: Strictly speaking, 12 A.M. denotes midnight, and 12 P.M. denotes noon, but there is sufficient confusion over these uses to make it advisable to use 12 noon and 12 midnight where clarity is required. A similar usage note can be found at <http://www.answers.com/topic/a-m>.

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 n" for noon and "12:00 m" for midnight, but this conflicts with the older tradition of using "12:00 m" for noon (Latin meridies), and "12:00 mn" for midnight (Latin media nox).

Still, use of 12:00 midnight is problematic because it does 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 favor 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, though some schedules now use the de facto convention.

The 24-hour clock notation avoids all of these ambiguities by using 00:00 for midnight at the start of the day and 12:00 for noon. From 23:59:59 the time shifts to 00:00:00, in fact the beginning of the next day. Some variants of 24-hour notation (including the world standard ISO 8601) explicitly use 24:00 when referring to midnight with reference to the previous day.

Advantages of the 12-hour clock

• Practically all analog clocks and watches have 12-hour dials.

• The 24-hour analog dials used by, for example, pilots and radio amateurs, have two and one half minutes to every hour, which can make them less intuitive to read than a 12-hour dial.

• Clocks with a chime are in step only on hours 1 to 12 and deviate for hour 0 and hours 13 to 23. Chiming 0 times is not a possible signal and chimes with more than 12 strikes tend to be difficult to count.

• Restricting the hour numbers to the range from 1 to 12 can accommodate a preference for monosyllabic words, which especially applies to the English language, in which ten of the 12 are monosyllabic, opposed to the 13-24 range, in which none are monosyllabic.

• Some people dislike the idea that midnight has two different notations in the 24-hour clock, depending on whether it is the beginning (00:00) or end (24:00) of the day.

• Some people find the idea of 0 or 00 hours unnatural.

• Some people claim that changing the notation of the clock destroys cultural identity.

• In terms of pronunciation, a specific 24-hour notation time such as "19:59", said as "nineteen fifty-nine", may sound too similar to and be mistaken as the calendar year 1959. While the years 2000 through 2009 and the last forty years in each century are exempt from such confusion, it does exist for the others.

Related conventions

Typography

The abbreviations "AM" and "PM" are variously written in small capitals ("am" and "pm"), 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.").

The use of a.m. as written in the form of am, AM, or A.M. can be confusing because am is an English word, AM is an abbreviation for amplitude modulation and A.M. is an abbreviation for anno mundi, in the year of the world, and master of arts.

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 and especially in computer applications in which it can help to maintain column alignment in tables and correct sorting order.

There are symbols for "a.m." (U+33C2 = "㏂") and "p.m." (U+33D8 = "㏘") 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

In informal language it is common to round a time to the nearest 5 minutes and express the time as so many minutes past an hour (eg 5:05 is five past 5) or minutes to an hour (eg 5:55 is five to six). The period 15 minutes is usually expressed as "quarter" (hence 5:15 is quarter past five) and 30 minutes is expressed as half (hence 5:30 is half past 5 or merely half five, the latter expression not being commonly used in the U.S.A.). For accurate times, the minutes may be expressed as an exact number of minutes past the hour specifying the time of day (eg 6:32 p.m. is thirty-two minutes past six in the evening).

Times of day ending in ":00" minutes (full hours) may be pronounced in English as the numbered hour followed by o'clock (10:00 as ten o'clock, 2:00 as two o'clock). This may be followed by the "a.m." or "p.m." designator, though phrases such as in the morning, in the afternoon, in the evening, or at night more commonly follow analog-style terms such as o'clock, half past three, and quarter to four. O'clock itself may be omitted, telling a time as four a.m. or four p.m. Minutes ":01" to ":09" are usually pronounced as oh one to oh nine (aught one to aught nine may still be in use in some Commonwealth countries). Minutes ":10" to ":59" are pronounced as their usual number-words. For instance, 6:02 a.m. can be pronounced six oh two a m; 6:32 a.m. could be told as six thirty-two a m.

In contexts in which the speaker has recently mentioned the hour of the day or for some other reason believes it to be well known to his or her hearers, he or she may omit all reference to it and simply declare the minutes, using expressions such as seventeen minutes past the (top of the) hour [to refer to 4:17 am, or 11:17 pm, etc.] or three minutes till the bottom of the hour [which similarly signals 7:57 pm, or 9:57 am, etc.]. This is also true of television broadcasts whose signals are picked up in more than one time zone, since the hour varies with those zones.

Military circles use the 24-hour clock exclusively and would typically pronounce times ending in :00 minutes as the hour followed by "hundred" with an optional "hours". For instance, 16:00 would be pronounced "sixteen hundred" or "sixteen hundred hours". In many such circumstances leading zeros would be voiced. For instance, 08:00 may be said as: "Oh eight hundred (hours)," with "oh" meaning the letter O, which is commonly spoken in place of "zero."

See also

• 24-hour clock
• Time
• Date and time notation by country
• Thai six-hour clock
• Decimal time
• ISO 8601

External links

• NIST FAQ on time
• 12am is noon in Japan