What is m 1200 and Why Do We Use Them?

Time counting system

"AM and PM" redirects here. For other uses, see AM PM (disambiguation)

"Ante meridiem" redirects here. For the meridian 180° both east and west of the prime meridian in a geographical coordinate system, see antimeridian

The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to "before midday") and p.m. (from Latin post meridiem, translating to "after midday").[1][2] Each period consists of 12 hours numbered: 12 (acting as 0),[3] 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. The 12-hour clock has been developed since the second millennium BC and reached its modern form in the 16th century.

The 12-hour time convention is common in several English-speaking nations and former British colonies, as well as a few other countries. There is no widely accepted convention for how midday and midnight should be represented: in English-speaking countries, "12 p.m." indicates 12 o'clock noon, while "12 a.m." means 12 o'clock midnight.[4][5][6]

History and use

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The natural day-and-night division of a calendar day forms the fundamental basis as to why each day is split into two cycles. Originally there were two cycles: one cycle which could be tracked by the position of the Sun (day), followed by one cycle which could be tracked by the Moon and stars (night). This eventually evolved into the two 12-hour periods which are used today, one called "a.m." starting at midnight and another called "p.m." starting at noon. Noon itself is rarely abbreviated today; but if it is, it is denoted "m."[1]

The 12-hour clock can be traced back as far as Mesopotamia and ancient Egypt.[7] Both an Egyptian sundial for daytime use[8] and an Egyptian water clock for night-time use were found in the tomb of Pharaoh Amenhotep I.[9] Dating to c. 1500 BC, these clocks divided their respective times of use into 12 hours each.

The Romans also used a 12-hour clock: daylight was divided into 12 equal hours (thus hours having varying length throughout the year) and the night was divided into four watches.[citation needed]

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 by their desire to model the Earth's apparent motion around 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, numbering was more likely to be based on the 24-hour system (I to XXIV). The 12-hour clock was used throughout the British empire.

During the 15th and 16th centuries, the 12-hour analog dial and time system gradually became established as standard throughout Northern Europe for general public use. The 24-hour analog dial was reserved for more specialized applications, such as astronomical clocks and chronometers.

Most analog clocks and watches today use the 12-hour dial, on which the shorter hour hand rotates once every 12 hours and twice in one day. Some analog clock dials have an inner ring of numbers along with the standard 1-to-12 numbered ring. The number 12 is paired either with a 00 or a 24, while the numbers 1 through 11 are paired with the numbers 13 through 23, respectively. This modification allows the clock to also be read in 24-hour notation. This kind of 12-hour clock can be found in countries where the 24-hour clock is preferred.

Use by country

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World map showing the usage of 12 or 24-hour clock in different countries

 

 24-hour

 

 24-hour (12-hour orally)

 

 Both in common use

 

 12-hour

In several countries the 12-hour clock is the dominant written and spoken system of time, predominantly in nations that were part of the former British Empire, for example, the United Kingdom, Republic of Ireland, the United States, Canada (excluding Quebec), Australia, New Zealand, South Africa, India, Pakistan, and Bangladesh, and others follow this convention as well, such as Mexico and the former American colony of the Philippines. In most countries, however, the 24-hour clock is the standard system used, especially in writing. Some nations in Europe and Latin America use a combination of the two, preferring the 12-hour system in colloquial speech but using the 24-hour system in written form and in formal contexts.

The 12-hour clock in speech often uses phrases such as ... in the morning, ... in the afternoon, ... in the evening, and ...at night. Rider's British Merlin almanac for 1795 and a similar almanac for 1773 published in London used them.[10] Other than in English-speaking countries and some Spanish-speaking countries, the terms a.m. and p.m. are seldom used and often unknown.

Computer support

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In most countries, computers by default show the time in 24-hour notation. Most operating systems, including Microsoft Windows and Unix-like systems such as Linux and macOS, activate the 12-hour notation by default for a limited number of language and region settings. This behaviour can be changed by the user, such as with the Windows operating system's "Region and Language" settings.[11]

Abbreviations

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Typical digital 12-hour alarm clock indicating p.m. with a dot to the left of the hour

The Latin abbreviations a.m. and p.m. (often written "am" and "pm", "AM" and "PM", or "A.M." and "P.M.") are used in English and Spanish.[12] The equivalents in Greek are π.μ. and μ.μ., respectively, and in Sinhala පෙ.ව. (pe.va.) for පෙරවරු (peravaru, පෙර pera – fore, pre) and ප.ව. (pa.va.) for පස්වරු (pasvaru, පස්සේ passē – after, post). However, noon is rarely abbreviated in either of these languages, noon normally being written in full. In Portuguese, there are two official options and many others used, for example, using 21:45, 21h45 or 21h45min (official ones) or 21:45 or 9:45 p.m. In Irish, a.m. and i.n. are used, standing for ar maidin ("in the morning") and iarnóin ("afternoon") respectively.

Most other languages lack formal abbreviations for "before noon" and "after noon", and their users use the 12-hour clock only orally and informally.[citation needed] However, in many languages, such as Russian and Hebrew, informal designations are used, such as "9 in the morning" or "3 in the night".

When abbreviations and phrases are omitted, one may rely on sentence context and societal norms to reduce ambiguity. For example, if one commutes to work at "9:00", 9:00 a.m. may be implied, but if a social dance is scheduled to begin at "9:00", it may begin at 9:00 p.m.

Related conventions

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Typography

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The terms "a.m." and "p.m." are abbreviations of the Latin ante meridiem (before midday) and post meridiem (after midday). Depending on the style guide referenced, the abbreviations "a.m." and "p.m." are variously written in small capitals ("am" and "pm"),[citation needed] uppercase letters without a period ("AM" and "PM"), uppercase letters with periods, or lowercase letters ("am" and "pm" or,[13] "a.m." and "p.m.").[citation needed] With the advent of computer generated and printed schedules, especially airlines, advertising, and television promotions, the "M" character is often omitted as providing no additional information as in "9:30A" or "10:00P".[14]

Some style guides suggest the use of a space between the number and the a.m. or p.m. abbreviation.[citation needed] Style guides recommend not using a.m. and p.m. without a time preceding it.[15]

The hour/minute separator varies between countries: some use a colon, others use a period (full stop),[13] and still others use the letter h.[citation needed] (In some usages, particularly "military time", of the 24-hour clock, there is no separator between hours and minutes.[16] This style is not generally seen when the 12-hour clock is used.)

Encoding

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Unicode specifies codepoints for "a.m." and "p.m." symbols, which are intended to be used only with Chinese-Japanese-Korean (CJK) character sets, as they take up exactly the same space as one CJK character:

• U+ 33C2

  ㏂

  SQUARE AM

• U+ 33D8

  ㏘

  SQUARE PM

Informal speech and rounding off

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In speaking, it is common to round the time to the nearest five minutes and/or express the time as the past (or to) the closest hour; for example, "five past five" (5:05). Minutes past the hour means those minutes are added to the hour; "ten past five" means 5:10. Minutes to, 'til and of the hour mean those minutes are subtracted; "ten of five", "ten 'til five", and "ten to five" all mean 4:50.

Fifteen minutes is often called a quarter hour, and thirty minutes is often known as a half hour. For example, 5:15 can be phrased "(a) quarter past five" or "five-fifteen"; 5:30 can be "half past five", "five-thirty" or simply "half five". The time 8:45 may be spoken as "eight forty-five" or "(a) quarter to nine".[17]

In older English, it was common for the number 25 to be expressed as "five-and-twenty".[18] In this way the time 8:35 may be phrased as "five-and-twenty to 9",[19] although this styling fell out of fashion in the later part of the 1900s and is now rarely used.[20]

Instead of meaning 5:30, the "half five" expression is sometimes used to mean 4:30, or "half-way to five", especially for regions such as the American Midwest and other areas that have been particularly influenced by German culture.[citation needed] This meaning follows the pattern choices of many Germanic and Slavic languages, including Serbo-Croatian, Dutch, Danish, Russian and Swedish, as well as Hungarian, Finnish and the languages of the Baltic States.

Moreover, in situations where the relevant hour is obvious or has been recently mentioned, a speaker might omit the hour and just say "quarter to (the hour)", "half past" or "ten 'til" to avoid an elaborate sentence in informal conversations. These forms are often commonly used in television and radio broadcasts that cover multiple time zones at one-hour intervals.[21]

In describing a vague time of day, a speaker might say the phrase "seven-thirty, eight" to mean sometime around 7:30 or 8:00. Such phrasing can be misinterpreted for a specific time of day (here 7:38), especially by a listener not expecting an estimation. The phrase "about seven-thirty or eight" clarifies this.

Some more ambiguous phrasing might be avoided. Within five minutes of the hour, the phrase "five of seven" (6:55) can be heard "five-oh-seven" (5:07). "Five to seven" or even "six fifty-five" clarifies this.

Formal speech and times to the minute

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"O'clock" redirects here. For the o'clock watch, see O bag

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 "six thirty-two"). Additionally, when expressing the time using the "past (after)" or "to (before)" formula, it is conventional to choose the number of minutes below 30 (e.g., 6:32 p.m. is conventionally "twenty-eight minutes to seven" rather than "thirty-two minutes past six").

In spoken English, full hours are often represented by 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 some 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 (nought or zero can also be used instead of oh). 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. whereas 6:32 a.m. could be told as six thirty-two a.m.

Confusion at noon and midnight

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It is not always clear what times "12:00 a.m." and "12:00 p.m." denote. From the Latin words meridies (midday), ante (before) and post (after), the term ante meridiem (a.m.) means before midday and post meridiem (p.m.) means after midday. Since "noon" (midday, meridies (m.)) is neither before nor after itself, the terms a.m. and p.m. do not apply.[2] Although "12 m." was suggested as a way to indicate noon, this is seldom done[27] and also does not resolve the question of how to indicate midnight.

The American Heritage Dictionary of the English Language states "By convention, 12 AM denotes midnight and 12 PM denotes noon. Because of the potential for confusion, it is advisable to use 12 noon and 12 midnight."[31]

E. G. Richards in his book Mapping Time (1999) provided a diagram in which 12 a.m. means noon and 12 p.m. means midnight.[32]

Historically, the style manual of the United States Government Printing Office used 12 a.m. for noon and 12 p.m. for midnight until its 2008 edition. At this point it reversed these designations[23][24] and then retained that change in its 2016 revision.[33]

Many U.S. style guides, and NIST's "Frequently asked questions (FAQ)" web page,[2] recommend 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."). The NIST website states that "12 a.m. and 12 p.m. are ambiguous and should not be used."

The Associated Press Stylebook specifies that midnight "is part of the day that is ending, not the one that is beginning."[30]

The Canadian Press Stylebook[28] says, "write noon or midnight, not 12 noon or 12 midnight." Phrases such as "12 a.m." and "12 p.m." are not mentioned at all.

In the UK, the National Physical Laboratory "FAQ-Time" web page[29] states "In cases where the context cannot be relied upon to place a particular event, the pair of days straddling midnight can be quoted"; also "the terms 12 a.m. and 12 p.m. should be avoided."

Likewise, some U.S. style guides recommend either clarifying "midnight" with other context clues, such as specifying the two dates between which it falls, or not referring to the term at all. For an example of the latter method, "midnight" is replaced with "11:59 p.m." for the end of a day or "12:01 a.m." for the start of a day. That has become common in the United States in legal contracts and for airplane, bus, or train schedules, though some schedules use other conventions. Occasionally, when trains run at regular intervals, the pattern may be broken at midnight by displacing the midnight departure one or more minutes, such as to 11:59 p.m. or 12:01 a.m.[34]

In Japanese usage, midnight is written as 午前0時 (0:00 a.m.) and noon is written as 午後0時 (0:00 p.m.), making the hours numbered sequentially from 0 to 11 in both halves of the day.

See also

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References

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AM and PM are terms used when writing times. However, there are a few different ways to write them, which can be tricky when proofreading. Should you just aim for consistency? Or is there a correct way to do it in certain contexts? In this post, we explore:

• What the terms AM and PM mean.
• How to capitalize and punctuate AM and PM.
• When to add a space between the time and AM or PM.
• What the most well-known style guides recommend.

Read on and learn more about writing the time. 

What Do These Terms Mean?

“AM” and “PM” are both abbreviations of Latin terms and refer to a specific time of day:

• AM (ante meridiem) means “before noon,” so it refers to the morning.
• PM (post meridiem) means “after noon,” so it refers to any time after midday.

When using a 12-hour clock, then, these terms clarify the time we have in mind (e.g., 12 AM is midnight, whereas 12 PM is midday). This is not necessary when using a 24-hour clock.

There are, however, a few options for how you write these terms, as we will explore below.

Capitalization of AM and PM

You may have noticed that we write “AM” and “PM” with capital letters in this post. For instance:

The performances today will be at 11 AM and 3 PM.

But that is not the only legitimate option! You can also write them with lowercase letters:

The performances today will be at 11 am and 3 pm.

Or you can write them with small caps. All are acceptable ways of writing these terms, so ultimately it comes down to preference or your client’s chosen style guide.

Punctuation of AM and PM

Another variation is adding periods between each letter in these terms. For instance:

The meeting ended at 11 a.m.

Did you see the 5:00 p.m. bulletin?

This is common when using lowercase letters, although some also add punctuation when using small caps. It would be unusual to punctuate these terms when using standard capital letters, though. Finally, keep in mind that you do not need an extra period after the abbreviation when punctuating a time like this at the end of a sentence.

Spacing AM and PM

The final variation you will see with these terms is whether to add a space between the time and AM/PM. In the examples so far, we have added this space, as it is more common to do so.

However, when using the unpunctuated forms of these terms, you can close the gap. This does not apply when using the punctuated forms, so take care not to mix up these styles:

Spaced and Unpunctuated: I have a class at 2 PM today. ✓

Unspaced and Unpunctuated: I have a class at 2PM today. ✓

Spaced and Punctuated: You need to be there by 9:00 a.m. ✓

Unspaced and Punctuated: You need to be there by 9:00a.m. ✗

As with the other options here, the space before AM and PM is largely a matter of preference.

What Do Style Guides Recommend?

As you can see, there are many options available when writing these terms. And as a proofreader, your main priority will be to ensure your client uses a consistent style. However, if your client is using a style guide, too, you can check it for advice.

Some well-known style guides suggest the following:

Style GuideLowercase or Capital?Punctuation?Spacing?AMASmall capsNoYesAPALowercaseYesYesAssociated PressLowercaseYesYesChicagoEither lowercase or small capsYes with lowercase letters; optional with small capsYesMHRA Lowercase NoNoMLA Lowercase YesYes

To learn more about proofreading for style, give our Becoming A Proofreader course a try.