12-hour clock

  1. ^ a b See Confusion
    at noon and midnight

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, translates to, before midday) and p.m. (from Latin post meridiem translates to, past midday).[1][2] Each period consists of 12 hours numbered: 12 (acting as zero),[3] 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 24 hour/day cycle starts at 12 midnight (may be indicated as 12 a.m.), runs through 12 noon (may be indicated as 12 p.m.), and continues to the midnight at the end of the day. The 12-hour clock has been developed from the middle of the second millennium BC to the 16th century AD.

The 12-hour time convention is common in several English-speaking nations and former British colonies, as well as a few other countries.

Clock system
12-hour 24-hour
Midnight (start of day)
12 midnight
12:00 a.m.[a]
12:01 a.m. 00:01
  1:00 a.m. 01:00
11:00 a.m. 11:00
11:59 a.m. 11:59
12 noon
12:00 p.m.[a]
12:01 p.m. 12:01
  1:00 p.m. 13:00
11:00 p.m. 23:00
11:59 p.m. 23:59
Midnight (end of day)
shown as start of next day

History and use

Exeter Cathedral astronomical clock
Exeter Cathedral Astronomical Clock, showing the double-XII numbering scheme

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 would eventually evolve into the two 12-hour periods that started at midnight (a.m.) and noon (p.m.) which are used today. 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.[4] Both an Egyptian sundial for daytime use[5] and an Egyptian water clock for night-time use were found in the tomb of Pharaoh Amenhotep I.[6] 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.

The first mechanical clocks in the 14th century, if they had dials at all, showed all 24 hours, used the 24-hour analog dial, influenced by astronomers' familiarity with the astrolabe and sundial, and 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, 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. 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 be read also in the 24-hour notation. This kind of 12-hour clock can be found in countries where the 24-hour clock is preferred.

Use by country

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, India, Pakistan, Bangladesh, Malaysia, Malta and others follow this convention as well such as Egypt, 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.[7] Other than English-speaking countries, the terms a.m. and p.m. are seldom used and often unknown.

Computer support

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 behavior can be changed by the user, such as with the Windows operating system "Region and Language" settings.[8]


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.[9] 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 any of these languages, noon normally being written in full. In Portuguese, there are two official options and many other 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. 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


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"), uppercase letters without a period ("AM" and "PM"), uppercase letters with periods, or lowercase letters ("am" and "pm" or, more commonly, "a.m." and "p.m.").

Some stylebooks suggest the use of a space between the number and the a.m. or p.m. abbreviation. Style guides recommend not using a.m. and p.m. without a time preceding it,[10] although doing so can be advantageous when describing an event that always happens before or after noon.

The hour/minute separator varies between countries: some use a colon, others use a period (full stop), and still others use the letter h. In many instances using the 24-hour clock, there is no separator between hours and minutes (0800, read as written, i.e. "oh-eight-hundred").


In Unicode, there exist symbols for:

"a.m." U+33C2 (HTML ㏂) and
"p.m." U+33D8 (HTML ㏘).

They are meant to be used only with Chinese-Japanese-Korean character sets, as they take up exactly the same space as one CJK character.

Informal speech and rounding off

It is common to round the time to the nearest five minutes and express the time as so many minutes past an hour, for example, "five past five" or "five to five". Fifteen minutes is often expressed as "a quarter" such as "a quarter past five" and 30 minutes as "half past five" or merely "half five"). The time of 8:45 may be spoken as either "eight forty-five" or "(a) quarter to nine".[11] Moreover, in situations where the relevant hour is obvious or has been recently mentioned, speakers can use terms "quarter to" and "half past" to avoid elaborate sentences in particularly informal conversations. These forms are often commonly used in television and radio broadcasts that cover multiple time zones at one-hour intervals.[12]

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. This meaning follows the pattern choices of many Germanic and Slavic languages, including Croatian, Dutch, Danish, Russian and Swedish, as well as Hungarian and Finnish.

Formal speech and times to the minute

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 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

Time according to various conventions
Device or style Midnight
Start of day
Noon Midnight
End of day
Written 24-hour time,
ISO 8601
00:00 12:00 24:00
Digital watches 12:00 AM 12:00 PM
U.S. Government Printing Office (1953)[13] midnight[a] noon
12 o'clock noon
12 m.
12:00 p.m.
U.S. Government Printing Office (2000)[14]  
12 a.m.
12 p.m.
U.S. Government Printing Office (2008)[15] 12 a.m.
12 midnight[a]
12 p.m.
12 noon
12 midnight[a]
Japanese legal convention[16] 0:00 a.m. 12:00 a.m. 12:00 p.m.
Chicago Manual of Style[17] noon
12:00 m.
Canadian Press,[18] UK standard[19] Midnight Noon Midnight
Associated Press style[20] noon midnight
NIST[2] midnight[b]
12:01 a.m.
noon midnight[b]
11:59 p.m.
  1. ^ a b c d e These styles are ambiguous with respect to whether midnight is at the start or end of each day.
  2. ^ a b NIST recommends using 11:59 p.m. and 12:01 a.m. to disambiguate when needed.
  1. ^ a b c d e These styles are ambiguous with respect to whether midnight is at the start or end of each day.
  2. ^ a b NIST recommends using 11:59 p.m. and 12:01 a.m. to disambiguate when needed.

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[17] 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."[21]

E. G. Richards in his book Mapping Time provided a diagram in which 12 a.m. means noon and 12 p.m. means midnight.[22] 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, when it reversed these designations,[14][15] later maintained in its 2016 revision.[23]

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."[20] Thus, according to AP style, "midnight Friday" occurs one minute after 11:59 p.m. Friday, not one minute before 12:01 a.m. Friday.

The Canadian Press Stylebook[18] 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. Britain's National Physical Laboratory "FAQ-Time" web page[19] 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 midnight 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.[24]

See also


  1. ^ a b "Time". The New Encyclopædia Britannica. 28. 1986. pp. 660 2a.
    "Time". Encyclopædia Britannica. Encyclopædia Britannica Online Library Edition. Retrieved 20 November 2013. (subscription required)
    "The use of AM or PM to designate either noon or midnight can cause ambiguity. To designate noon, either the word noon or 1200 or 12 M should be used. To designate midnight without causing ambiguity, the two dates between which it falls should be given unless the 24-hour notation is used. Thus, midnight may be written: May 15–16 or 2400 May 15 or 0000 May 16."
  2. ^ a b c d "Times of Day FAQs". National Institute of Standards and Technology. 21 September 2016. Retrieved 30 September 2018.
  3. ^ Susan Addington (25 August 2016). "Modular Arithmetic". Archived from the original on 4 July 2008. Retrieved 28 November 2008.
  4. ^ "The History of Clocks". 13 October 2008. Archived from the original on 13 October 2008. Retrieved 12 November 2017.
  5. ^ "Berlin instruments of the old Eg.time of day destination". members.aon.at.
  6. ^ A Walk through Time - Water Clocks Archived 31 May 2008 at the Wayback Machine
  7. ^ National Library of Australia catalogue entry for Rider's British merlin: for the year of Our Lord God 1795
  8. ^ Lawrence Abrams (13 December 2012). "How to customize how the time is displayed in Windows". Bleeping Computer. Retrieved 26 October 2013.
  9. ^ Diccionario panhispánico de dudas, HORA (in Spanish)
  10. ^ Hacker, Diana, A Writer's Reference, six edition, Bedford, St Martin's, Boston, 2007, section M4-c, p.308.
  11. ^ American Heritage Dictionary, 3rd ed. (Boston: Houghton Mifflin, 1992). s.v. usage note at end of "quarter" entry.
  12. ^ "TVTimes". 21–27 May 1983. Retrieved 22 April 2012.
  13. ^ "United States Government Printing Office Style Manual". January 1953. pp. 152 and 267.
  14. ^ a b "U.S. Government Printing Office Style Manual" (PDF). page 156.
  15. ^ a b "U.S. Government Printing Office Style Manual - Chapter 12 - Numerals". www.gpo.gov.
  16. ^ "午前12時? 午後0時?" [12 AM? or 0 PM?]. National Institute of Information and Communications Technology (in Japanese). 15 February 1989. Retrieved 24 May 2017.
  17. ^ a b Chicago Manual of Style (17th ed.). University of Chicago Press. 2017. paragraph 9.38. ISBN 978-0-226-28705-8. Although noon can be expressed as 12:00 m. (m = meridies), very few use that form.
  18. ^ a b The Canadian Press Stylebook (11th ed.). 1999. page 288.
  19. ^ a b National Physical Laboratory, FAQ-Time
  20. ^ a b Ed. Norm Goldstein, The Associated Press Stylebook and Briefing on Media Law: with Internet Guide and Glossary, P.161, 177, Perseus Publishing, 2002, Cambridge, Massachusetts, United States, LCCN 2002105974, ISBN, 0-7382-0740-3
  21. ^ AM at the American Heritage Dictionary of the English Language, Fifth Edition (2011)
  22. ^ Richards, E. G., Mapping Time: the Calendar and its History (Oxford University Press, 1999), 289.
  23. ^ "New Edition of the GPO Style Manual - govinfo". www.govinfo.gov.
  24. ^ Interim train timetables Archived 26 May 2015 at the Wayback Machine, Abellio Greater Anglia, London, 17 May 2015, pages 7 and 8.

External links

11 A.M.

11 A.M. is a time on the 12-hour clock.

11 A.M. may also refer to:

11 A.M. (film), a 2013 South Korean film

11AM (TV series), an Australian TV news program that aired from 1982 to 1999

"11AM", a song by Incubus from their 2001 album Morning View

12 o'clock

12 O'clock usually refers the time as shown on a 12-hour clock, either noon - 12 o'clock at daytime - or midnight - 12 o'clock at nighttime.

It may also refer to:

The corresponding clock position, straight ahead or directly above

A Wu-Tang Clan affiliate, 12 O'Clock

A community in Quinte West

12 O'Clock (1958 film), a Bollywood film starring Guru Dutt and Waheeda Rehman

12 O'Clock (in two parts), track in Heaven and Hell, a studio album by Greek electronic composer Vangelis

1 A.M.

1 A.M. or 1am or variation may refer to:

A time on the 12-hour clock

1 am = 1 attometre (a very small distance)

Orders of magnitude (length), a comparative scale regime at 1 attometer

Aircraftman, 1st class (1AM) in the Royal New Zealand Air Force

24-hour clock

The 24-hour clock is the convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today, and is used by international standard ISO 8601.A limited number of countries, particularly English-speaking, use the 12-hour clock, or a mixture of the 24- and 12-hour time systems. In countries where the 12-hour clock is still dominant, some professions prefer to use the 24-hour clock. For example, in the practice of medicine the 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patient's medical history. In the United States and a handful of other countries, it is popularly referred to as military time.

2 A.M.

2 A.M. may refer to:

A time on the 12-hour clock

4 P.M.

4 P.M. may refer to:

A time on the 12-hour clock

4 P.M. (group)

"4 P.M.", song from Stand Back! Here Comes Charley Musselwhite's Southside Band

5 A.M.

5 A.M. may refer to:

A time on the 12-hour clock

5 P.M.

5 P.M. or variants may refer to:

A time on the 12-hour clock

5 PM (film), a 2017 Iranian comedy film

5 o'clock

5 o'clock or five o'clock may refer to:

"5 O'Clock" (T-Pain song), 2011 R&B song by T-Pain

"5 O'Clock" (Nonchalant song), 1996 hip-hop song by Nonchalant

5th hour of a clock, see 12-hour clock

The end of a 9-to-5 workday, see Working time

The 5 O'Clock Show, 2010 UK Channel 4 TV talk show

Richard Hammond's 5 O'Clock Show, 2006 UK ITV TV talk show

It's Five O'Clock, 1969 album by Aphrodite's Child

"It's Five O'Clock" (song), a song by Aphrodite's Child from the 1969 album It's Five O'Clock

Clock position

A clock position is the relative direction of an object described using the analogy of a 12-hour clock to describe angles and directions. One imagines a clock face lying either upright or flat in front of oneself, and identifies the twelve hour markings with the directions in which they point.

Using this analogy, 12 o'clock means ahead or above, 3 o'clock means to the right, 6 o'clock means behind or below, and 9 o'clock means to the left. The other eight hours refer to directions that are not directly in line with the four cardinal directions.

In aviation, a clock position refers to a horizontal direction; it may be supplemented with the word high or low to describe the vertical direction which is pointed towards your feet. 6 o'clock high means behind and above the horizon, while 12 o'clock low means ahead and below the horizon.

Date and time notation in Croatia

Croatia uses the day-month-year date notation and both the 24-hour and the 12-hour clock for expressing time of day.

Date and time notation in Turkey

In Turkey, the little endian date format is used, and 24-hour clock is more common than 12-hour clock system.

Date and time representation by country

Different conventions exist around the world for date and time representation, both written and spoken.

Italian six-hour clock

The six-hour clock (Italian: sistema orario a 6 ore), also called the Roman (alla romana) or the Italian (all'italiana) system, is a timekeeping system used in Italy. In this system, the day starts at the evening Ave Maria at the end of twilight, approximately half an hour after sunset, and the following 24 hours are divided into four cycles of six hours each.

Introduced by the Catholic Church in the 13th century, it remained in use in Italy until superseded by the 12-hour clock following the Napoleonic invasion of Italy.

Striking clock

A striking clock (also known as chiming clock) is a clock that sounds the hours audibly on a bell or gong. In 12-hour striking, used most commonly in striking clocks today, the clock strikes once at 1:00 A.M., twice at 2:00 A.M., continuing in this way up to twelve times at 12:00 P.M., then starts again, striking once at 1:00 P.M., twice at 2:00 P.M., up to twelve times at 12:00 A.M.

The striking feature of clocks was originally more important than their clock faces; the earliest clocks struck the hours, but had no dials to enable the time to be read. The development of mechanical clocks in 12th century Europe was motivated by the need to ring bells upon the canonical hours to call the community to prayer. The earliest known mechanical clocks were large striking clocks installed in towers in monasteries or public squares, so that their bells could be heard far away. Though an early striking clock in Syria was a 12-hour clock, many early clocks struck up to 24 strokes, particularly in Italy, where the 24-hour clock, keeping Italian hours, was widely used in the 14th and 15th centuries. As the modern 12-hour clock became more widespread, particularly in Great Britain and Northern Europe, 12-hour striking became more widespread and eventually became the standard. In addition to striking on the hour, many striking clocks play sequences of chimes on the quarter-hours. The most common sequence is Westminster Quarters.

Today the time-disseminating function of clock striking is almost no longer needed, and striking clocks are kept for historical, traditional, and aesthetic reasons. Historic clock towers in towns, universities, and religious institutions worldwide still strike the hours, famous examples being Big Ben in London, the Peace Tower in Ottawa, and the Kremlin Clock in Moscow. Home striking clocks, such as mantel clocks, cuckoo clocks, grandfather clocks and bracket clocks are also common.

A typical striking clock will have two gear trains, because a striking clock must add a striking train that operates the mechanism that rings the bell in addition to the timekeeping train that measures the passage of time.

Time in Ethiopia

The time zone in Ethiopia is East Africa Time (EAT) (UTC+03). The IANA time zone database identifier is "Africa/Addis Ababa." Ethiopia does not observe daylight saving time. Almost all Ethiopians use a 12-hour clock system. The daytime cycle begins at dawn 12:00 (6:00:00 AM EAT) and ends at dusk 11:59:59 (5:59:59 PM EAT).

The night time cycle begins at dusk 12:00 (6:00:00 PM EAT) and ends at dawn 11:59:59 (5:59:59 AM EAT).

The convention is that the day begins at 1:00 o'clock in the morning 12 hour cycle (7:00 AM EAT) rather than midnight (12:00 AM EAT). The current time convention persists despite inroads of international norms.

Key concepts
Measurement and
  • Religion
  • Mythology
Philosophy of time
Human experience
and use of time
Time in
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Obsolete standards
Time in physics
Archaeology and geology
Astronomical chronology
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