12-hour clock

The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided, or seperated 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. 12 noon and 12 midnight can more clearly and correctly express these times.

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^[1] and an Egyptian water clock for nighttime use found in the tomb of Pharaoh Amenhotep I,^[2] 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 XXIIII), 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.

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:

• Australia
• Canada (except Quebec)
• United Kingdom and other English-speaking regions
• United States of America
• New Zealand
• Philippines

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

• Albania
• Bulgaria
• Greece
• Quebec, Canada
• Turkey
• some Spanish-speaking regions of South America

In many European countries, a 12-hour clock is commonly used in informal speech, but no concept of a.m./p.m. exists. 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.

Use by country

Main article: date and time notation by country

Abbreviations

The Latin abbreviations "a.m." and "p.m." (often spelled "am" and "pm") or "AM" and "PM" 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. However, even most people who grew up with the 12-hour clock do not know how to use it to indicate midnight and noon. In addition, most of these people 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 "12:46am" appears unintuitively and seemingly illogically before a message marked "11:05am" for example.

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 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;
• generally more complicated to understand and to implement in software and digital electronics.

Confusion at noon and midnight

The abbreviations for the terms ante meridiem (before noon) and post meridiem (after noon) should not be applied to noon and 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-hour 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 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).

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

• Most analog clocks and watches have 12 hour dials. The 24 hour analog dials used by, for example, pilots and radio amateurs, are less commonly seen, and consequently require some practice to read. Children learn only the 12 hour analog dial at school.

• Some 12 hour analog dials try to show the hours from 13 to 24 on an additional ring, usually inside the main 1 to 12 ring. This is an inelegant solution.

• 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 chimes with more than 12 strikes tend to be difficult to count and annoying.

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

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

• 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.

Related conventions

Typography

The initialisms "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.").

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 it can help to maintain column alignment in tables and correct sorting order.

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). 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). 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).

There are symbols for "a.m." (㏂ = "㏂") and "p.m." (㏘ = "㏘") 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 (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 or may be not if obvious. 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 (ought one to ought 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.

References

1. ^ Berlin instruments of the old Egyptian time of day destination
2. ^ A Walk through Time - Water Clocks

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
• 12 hour dials

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