12-HOUR CLOCK

12-hour clock	24-hour clock
midnight or 12:00 a.m.	00:00
1:00 a.m.	01:00
2:00 a.m.	02:00
3:00 a.m.	03:00
4:00 a.m.	04:00
5:00 a.m.	05:00
6:00 a.m.	06:00
7:00 a.m.	07:00
8:00 a.m.	08:00
9:00 a.m.	09:00
10:00 a.m.	10:00
11:00 a.m.	11:00
noon or 12:00 p.m.	12:00
1:00 p.m.	13:00
2:00 p.m.	14:00
3:00 p.m.	15:00
4:00 p.m.	16:00
5:00 p.m.	17:00
6:00 p.m.	18:00
7:00 p.m.	19:00
8:00 p.m.	20:00
9:00 p.m.	21:00
10:00 p.m.	22:00
11:00 p.m.	23:00
undefined (midnight)	24:00

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 day", idiomatically "before the middle of the day") and '''post meridiem''' ('''p.m.''', "after the middle of the day"). Each period consists of 12 hours numbered 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Hourly time periods run 12:00 through 11:59 either ante meridiem or post meridiem.^[1] When referencing twelve o'clock the descriptor is either noon or midnight.

Contents

History and usage

Use by country

Abbreviations

Criticism and practical problems

Confusion at noon and midnight

Advantages of the 12-hour clock

Related conventions

Typography

Pronunciation

References

See also

External links

History and usage

The 12-hour clock can be traced back as far Mesopotamia and Egypt but may also have roots in India. However, the lengths of the ancient hours varied seasonally, always with 12 hours from sunrise to sunset and 12 hours from sunset to sunrise. In Egypt the hour beginning and ending each half-day (four hours each day) were considered twilight hours. An Egyptian sundial for daylight use^[2] and an Egyptian water clock for nighttime use found in the tomb of Pharaoh Amenhotep I,^[3] 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.

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
:
★ France
:
★ Greece
:
★ Quebec, Canada
:
★ Turkey
:
★ Ireland
:
★ 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

★ The rollover from 12 to 1 happens an hour later than the change between a.m. and p.m.

★ The lexicographical order does not match the chronological order.

★ It's generally more complicated to understand and to implement in software and digital electronics.

★ Typographically, the a.m. and p.m. suffixes require more space.

Confusion at noon and midnight

Style	Midnight (start of day)	Noon	Midnight (end of day)
24-hour clock, ISO 8601	00:00	12:00	24:00
Most digital 24-hour clocks	00:00	12:00	—
12-hour digital clocks indicating a.m./p.m.	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.
UK standard	Midnight	Noon	Midnight
NIST1	Midnight	Noon	Midnight
NIST2	12:00 Midnight	12:00 Noon	12:00 Midnight
U.S. de facto legal	12.01 a.m.	—	11.59 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 the United States to designate 12:00 p.m. as noon and 12:00 a.m. as midnight at the beginning of the day. With this convention the change from a.m. to p.m. (and vice versa) coincides with the change of hour from 11 to 12. This convention is standardized for computer usage in an ANSI standard.
12 a.m. and 12 p.m. are occasionally used in the UK, but with no consistent meaning, and such terms are generally considered to be ambiguous and incorrect, for reasons explained in the website of the National Maritime Museum, Greenwich:

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-".

http://www.nmm.ac.uk/server/show/conWebDoc.17917
However, it should be noted that the Sun is at its highest point in the sky at noon, apparent solar time, not standard time. (See . Note that apparent solar time is also known as local solar time.)
The use of 12:00 a.m. for midnight and 12:00 p.m. for noon is contrary to the ''U.S. Government Printing Office Style Manual''^[4] which recommends the use of 12 p.m. for midnight and 12 a.m. formerly 12 m. for noon.
On the other hand, the American Heritage Dictionary (Second College Edition) 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 .
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,^[5] but this conflicts with the older tradition of using "12:00 " for noon (Latin ''meridies''), and "12:00 " 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 use other conventions.
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 (one second later) 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 when referring to midnight at the end of the current or referenced day.

Advantages of the 12-hour clock

Main articles: Comparison of the 12-hour and 24-hour clocks

Some notable advantages to the use of the 12-hour clock are:

★ It correponds to analog clocks, which almost all have 12-hours.

★ It's always possible to make the distinction between a.m. and p.m. in writing, whereas in isolation a time of "11:00" with no suffix as with the 24-hour clock might be in the morning or evening (however, this is paradoxically caused by the existence of 12-hour notation)

★ The use of a leading zero, as is often used for 00:00 to 09:59, is awkward to pronounce out loud.

Related conventions

Typography

The abbreviations "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.").
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 for Master of Arts.
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 five minutes and express the time as so many minutes past an hour (e.g., 5:05 is five past five) or minutes to an hour (e.g., 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 five 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 (e.g., 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 the bottom half of the clock, such as 7:27 pm, or 9:27 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."

References

1. National Institute of Standards and Technology's Physics Laboratory, Time and Frequency Division
2. Berlin instruments of the old Egyptian time of day destination
3. A Walk through Time - Water Clocks
4. United States Government Printing Office Style Manual 2000, Section 9.54: References to Meridian in Statements of Time
5. Wisconsin Occupational Operator License Application

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