12-hour clock
Encyclopedia
| Clock system | |
|---|---|
| 12-hour | 24-hour 24-hour clock The 24-hour clock is a 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... |
| midnight (start of day) 12:00* |
00:00 |
| 12:59 a.m. | 00:59 |
| 1:00 a.m. | 01:00 |
| 2:00 a.m. | 02:00 |
| … | … |
| 10:00 a.m. | 10:00 |
| 11:00 a.m. | 11:00 |
| 11:59 a.m. | 11:59 |
| 12:00 noon* | 12:00 |
| 12:01 p.m. | 12:01 |
| 12:59 p.m. | 12:59 |
| 1:00 p.m. | 13:00 |
| 2:00 p.m. | 14:00 |
| … | … |
| 10:00 p.m. | 22:00 |
| 11:00 p.m. | 23:00 |
| 11:59 p.m. | 23:59 |
| midnight (end of day) shown as start of next day |
24:00 |
| * See section "Confusion at noon and midnight" |
|
The 12-hour clock is a time conversion convention in which the 24 hour
Hour
The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
s of the day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
are divided into two periods called ante meridiem (a.m., English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...
: "before midday") and post meridiem (p.m., English: "after midday"). Each period consists of 12 hours numbered: 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11.
The 12-hour clock was developed over time from the mid-second millennium BC
Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...
to the 16th century AD
Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...
.
History and use
Mesopotamia
Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...
and Ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...
: Both an Egyptian sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...
for daytime use and an Egyptian water clock
Water clock
A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured.Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions...
for night time use were found in the tomb of Pharaoh Amenhotep I
Amenhotep I
Amenhotep I was the second Pharaoh of the 18th dynasty of Egypt. His reign is generally dated from 1526 to 1506 BC. He was born to Ahmose I and Ahmose-Nefertari, but had at least two elder brothers, Ahmose-ankh and Ahmose Sapair, and was not expected to inherit the throne...
. Dating to c. 1500 BC, these clocks divided their respective times of use into 12 hours each.
The Romans
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....
also used a 12-hour clock: daylight was divided into 12 equal hours (of, thus, varying length throughout the year) and the night was divided into four 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 usage of "three hours into the first 12-hour period of the day".
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
Astrolabe
An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...
and sundial, and their desire to model the apparent motion of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. In Northern Europe
Northern Europe
Northern Europe is the northern part or region of Europe. Northern Europe typically refers to the seven countries in the northern part of the European subcontinent which includes Denmark, Estonia, Latvia, Lithuania, Norway, Finland and Sweden...
these dials generally used the 12-hour numbering scheme
Numbering scheme
There are many different numbering schemes for assigning nominal numbers to entities. These generally require an agreed set of rules, or a central coordinator. The schemes can be considered to be examples of a primary key of a database management system table, whose table definitions require a...
in Roman numerals
Roman numerals
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...
, 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
Wells Cathedral clock
The Wells Cathedral clock is an astronomical clock in the north transept of Wells Cathedral, England.The clock is one of the group of famous 14th to 16th century astronomical clocks to be found in the West of England....
and Exeter. Elsewhere in Europe, particularly in Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, 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. The 24-hour analog dial was reserved for more specialized applications, such as astronomical clock
Astronomical clock
An astronomical clock is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.-Definition:...
s and chronometers.
Most analog clocks and watches today use the 12-hour dial, on which the shorter and sometimes thicker hour hand rotates once every 12 hours and twice in one day. Some analog clock dials have an inner ring of numbers paired with the standard 1 to 12 ring of numbers. 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 the 24-hour notation. The 12-hour clock can be found in countries where the 24-hour clock is preferred.
Use 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:
- AustraliaAustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
- BangladeshBangladeshBangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...
- CanadaCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
(excluding QuebecQuebecQuebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, in French) - ColombiaColombiaColombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...
- EgyptEgyptEgypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
- El SalvadorEl SalvadorEl Salvador or simply Salvador is the smallest and the most densely populated country in Central America. The country's capital city and largest city is San Salvador; Santa Ana and San Miguel are also important cultural and commercial centers in the country and in all of Central America...
- HondurasHondurasHonduras is a republic in Central America. It was previously known as Spanish Honduras to differentiate it from British Honduras, which became the modern-day state of Belize...
- Hong KongHong KongHong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...
- IndiaIndiaIndia , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
- IranIranIran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
- IrelandIrelandIreland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
- JordanJordanJordan , officially the Hashemite Kingdom of Jordan , Al-Mamlaka al-Urduniyya al-Hashemiyya) is a kingdom on the East Bank of the River Jordan. The country borders Saudi Arabia to the east and south-east, Iraq to the north-east, Syria to the north and the West Bank and Israel to the west, sharing...
- Malaysia
- MexicoMexicoThe United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
- New ZealandNew ZealandNew Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
- NicaraguaNicaraguaNicaragua is the largest country in the Central American American isthmus, bordered by Honduras to the north and Costa Rica to the south. The country is situated between 11 and 14 degrees north of the Equator in the Northern Hemisphere, which places it entirely within the tropics. The Pacific Ocean...
- PakistanPakistanPakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
- PhilippinesPhilippinesThe Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
- Saudi ArabiaSaudi ArabiaThe Kingdom of Saudi Arabia , commonly known in British English as Saudi Arabia and in Arabic as as-Sa‘ūdiyyah , is the largest state in Western Asia by land area, constituting the bulk of the Arabian Peninsula, and the second-largest in the Arab World...
- SingaporeSingaporeSingapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...
- United KingdomUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
- United StatesUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
- VenezuelaVenezuelaVenezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
- NigeriaNigeriaNigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...
- GhanaGhanaGhana , officially the Republic of Ghana, is a country located in West Africa. It is bordered by Côte d'Ivoire to the west, Burkina Faso to the north, Togo to the east, and the Gulf of Guinea to the south...
- Australia
In most other countries the 12-hour clock is used in speech alongside the 24-hour clock
24-hour clock
The 24-hour clock is a 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...
.
In many European countries and Western countries, the 12-hour clock is commonly used in informal speech with descriptive phrases such as in the morning, in the afternoon, in the evening, and at night. Outside of English-speaking countries, the terms a.m. and p.m. are seldom used and often unknown. In England, the descriptive phrases were
universal until comparatively recently - Rider's British Merlin almanack for 1795 (published in London) uses them, and so does a similar almanack for 1773.
Abbreviations
The LatinLatin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
abbreviations a.m. and p.m. (often written "am" and "pm", "AM" and "PM", or "A.M." and "P.M.") are used in English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...
and Spanish
Spanish language
Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...
. The equivalents in Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
are π.µ. and µ.µ., 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.
In practice, when abbreviations and phrases are omitted, one relies on sentence context and common sense to distinguish the meaning. For example, if one schedules an appointment with a doctor at "9:00" on a certain date, that means 9:00 a.m.; but if a social dance
Social dance
Social dance is a major category or classification of danceforms or dance styles, where sociability and socializing are the primary focuses of the dancing...
is scheduled to begin at "9:00", it means 9:00 p.m.
Typography
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 guideStyle guide
A style guide or style manual is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization or field...
referenced, the abbreviations "AM" and "PM" are variously written in small capitals ("" and ""), uppercase letters ("AM" and "PM"), or lowercase letters ("am" and "pm").
There are symbols for "a.m." (U+33C2 = "㏂") and "p.m." (U+33D8 = "㏘") in Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
. They are meant to be used only with Chinese-Japanese-Korean character sets, however, as they take up exactly the same space as one Chinese character
Chinese character
Chinese characters are logograms used in the writing of Chinese and Japanese , less frequently Korean , formerly Vietnamese , or other languages...
.
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, 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).
Informal speech and rounding off
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 "five oh five") or minutes to an hour (e.g., 5:55 is "five to six"). The period 15 minutes is often expressed as "a quarter" (hence 5:15 is "a 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 common in the USA). The time 8:45 is spoken as "(a) quarter to (or of, before, or til) nine". Moreover, in situations where the relevant hour is obvious or has been recently mentioned, speakers can state simply "quarter to", "half past", etc., to avoid elaborate sentences in particularly informal conversations.Instead of meaning 5:30, the "half five" convention is sometimes used to mean 4:30, i.e., "half-way to five", especially in the more German-influenced parts
German American
German Americans are citizens of the United States of German ancestry and comprise about 51 million people, or 17% of the U.S. population, the country's largest self-reported ancestral group...
of the U.S.A (the Midwest, essentially). "Half-way to five" follows the usage in German speaking countries. It is also found in Indonesian
Indonesian language
Indonesian is the official language of Indonesia. Indonesian is a normative form of the Riau Islands dialect of Malay, an Austronesian language which has been used as a lingua franca in the Indonesian archipelago for centuries....
, Norwegian, Danish, Swedish, Dutch, Serbian, Estonian, Latvian, Lithuanian, Polish, Czech, Slovak, Finnish, Hungarian, Russian and Afrikaans.
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 "thirty-two minutes past six in the evening").Times of day ending in ":00" minutes (full hours) are often said 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 (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; 6:32 a.m. could be told as six thirty-two a.m.
Confusion at noon and midnight
| Time as denoted by various devices or styles | |||
|---|---|---|---|
| Device or style Style guide A style guide or style manual is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization or field... |
Midnight Midnight Midnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons.... start of day |
Noon Noon Noon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break... |
Midnight end of day |
| Written 24-hour time, including ISO 8601 ISO 8601 ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization and was first published in 1988... |
00:00 | 12:00 | 24:00 |
| 24-hour digital clock Digital clock A digital clock is a type of clock that displays the time digitally, i.e. in cyphers, as opposed to an analog clock, where the time is displayed by hands. Digital clocks are often associated with electronic drives, but the "digital" description refers only to the display, not to the drive mechanism... s |
00:00 | 12:00 | — * |
| 12-hour digital clocks with a.m. and p.m. |
12:00 a.m. | 12:00 p.m. | — * |
| Written 12-hour time | ** | ** | — ** |
| U.S. Government Printing Office | 12 a.m. | 12 p.m. | — |
| U.S. Government Printing Office (1953) | — | 12:00 a.m. | 12:00 p.m. |
| Japanese legal conventionJapanese Page | 0:00 a.m. | 12:00 a.m. | 12:00 p.m. |
| Antiquated † | 12:00 m.n. | 12:00 m. | 12:00 m.n. |
| Canadian Press Canadian Press Canadian Press Enterprises Inc. is the entity which "will take over the operations of the Canadian Press" according to a November 26, 2010 article in the Toronto Star... , UK standard, NIST1 † |
midnight | noon | midnight |
| NIST2 † | 12:00 Midnight | 12:00 Noon | 12:00 Midnight |
| Associated Press Style | 12:01 a.m. | noon | — |
| U.S. de facto De facto De facto is a Latin expression that means "concerning fact." In law, it often means "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established." It is commonly used in contrast to de jure when referring to matters of law, governance, or... legal |
12:01 a.m. | — | 11:59 p.m. |
| Encyclopædia Britannica | Midnight December 11–12 |
12m | Midnight December 12–13 |
| * Digital clocks never reach midnight at the end of the day. Instead they wrap from 11:59 p.m. or 23:59 to midnight at the start of the next day. Likewise the written 12-hour style wraps immediately to the start of the next day. † These styles are ambiguous with respect to whether midnight is at the start and or end of each day. |
|||
Since the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
word meridies means noon or midday, it is inconsistent to refer to noon as either "12 a.m." ("12 ante meridiem", or "12 o'clock before noon") or as "12 p.m." ("12 post meridiem", or "12 o'clock after noon"). On the other hand, midnight could logically be called either "12 p.m." (12 post meridiem, 12 hours after the previous noon) or "12 a.m." (12 ante meridiem, 12 hours before the following noon); "x a.m." no longer means "x hours before noon", but the x-numbered hour before noon.
The National Maritime Museum
National Maritime Museum
The National Maritime Museum in Greenwich, England is the leading maritime museum of the United Kingdom and may be the largest museum of its kind in the world. The historic buildings forming part of the Maritime Greenwich World Heritage Site, it also incorporates the Royal Observatory, Greenwich,...
, Greenwich, states:
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-".
However, as discussed elsewhere in the same reference, the sun is highest at 12 noon local Solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...
, not 12 noon civil time, the difference being given by the equation of time
Equation of time
The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...
plus the effect of time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...
s and of the daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...
.
The American Heritage Dictionary of the English Language
The American Heritage Dictionary of the English Language
The American Heritage Dictionary of the English Language is an American dictionary of the English language published by Boston publisher Houghton Mifflin, the first edition of which appeared in 1969...
(Fourth Edition, 2000) has a similar usage note on this topic: "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."
Many U.S. style guides (including the NIST website) 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.", respectively). Some other style guides suggest "12:00 " for noon and "12:00 " for midnight, but that conflicts with the older tradition of using "12:00 " for noon (Latin meridies), and "12:00 " for midnight (Latin media nox).
The Canadian Press Stylebook (11th Edition, 1999, page 288) 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.
The use of "12:00 midnight" or "midnight" is still problematic because it does not distinguish between the midnight at the start of a particular day and the midnight at its end. To avoid confusion and error, some U.S. style guides recommend either clarifying "midnight" with other context clues, 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 the next day. That has become common in the United States in legal contracts and for airplane
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
, bus
Bus
A bus is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are...
, or train
Train
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...
schedules, though some schedules use other conventions.
The 24-hour clock
24-hour clock
The 24-hour clock is a 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...
notation avoids all of those 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, the beginning of the next day. In 24-hour notation 24:00 can be used to refer to midnight at the end of a day.
In Britain, various conventions are employed. TV mag, "Sun" newspaper, London, 17 December 2005 uses "noon (12.00)" and "midnight (0.00)" in individual listings. Sequential listings start with a.m. or p.m. as appropriate, but these indicators are not used again, although in sub-listings "12midnight" is sometimes employed. The London Daily Telegraph uses "12.00noon" and "12.00midnight" in individual listings. In sequential listings the first programme to start after 12.00 is marked "am" or "pm" as appropriate. If a station comes on - air at 12.00 the time is marked "12 00 noon". If it goes off - air at 12.00 the time is marked "12.00 midnight".
See also
- 24-hour clock24-hour clockThe 24-hour clock is a 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...
- Clock positionClock positionClock position is a way of communicating the relative direction of an object using the analogy of a 12-hour clock. 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...
- Date and time notation by countryDate and time notation by countryDifferent conventions exist around the world for date and time representation, both written and spoken.- Differences :Differences can exist in:*The calendar that is used.*The order in which the year, month and day are represented....
- Decimal timeDecimal timeDecimal time is the representation of the time of day using units which are decimally related. This term is often used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal...
- MidnightMidnightMidnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....
- NoonNoonNoon is usually defined as 12 o'clock in the daytime. The word noon is also used informally to mean midday regarding the location of the sun not the middle of a persons day. Although this is a time around the middle of the day when people in many countries take a lunch break...
- Thai six-hour clockThai six-hour clockThe six-hour clock is a traditional timekeeping system used in Thai and Lao language alongside the official 24-hour clock. Like the other common systems, it counts twenty-four hours in a day, but divides the day into four quarters, counting six hours in each. The hours in each quarter are told...