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.
The 12-hour clock can be traced back as far as Mesopotamia and Ancient Egypt. Both an Egyptian sundial for daytime use and an Egyptian water clock for night-time use were found in the tomb of Pharaoh Amenhotep I. 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.
In several countries the 12-hour clock is the dominant written and spoken system of time. Other countries use the 12-hour clock mainly in spoken time, while the 24-hour notation is written. In most countries the 12-hour clock is used in speech alongside the 24-hour clock.
The 12-hour clock in speech often uses 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 relatively recently; e.g., Rider's British Merlin almanac for 1795 (published in London) uses them, and so does a similar almanac for 1773.
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.
The behaviour can be changed by the user, for example in Windows 7 "Region and Language" settings. In Windows, AM/PM notations are independent of 12 or 24 hour numbering, allowing the clock to display 16:18:12 PM, for example.
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. 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 pm): 21h45 or 21h45min (official ones) or 21:45 or 9:45 p.m.
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 schedules an appointment with a doctor at "9:00", one may mean 9:00 am, but if a social dance is scheduled to begin at "9:00", it may begin at 9:00 p.m.
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 ("AM" and "PM"), or lowercase letters ("am" and "pm").
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), and still others use the letter h. In many instances using the 24-hour clock, there is no separator between hours and minutes (0800, pronounced as if spelled "oh-eight-hundred").
There are symbols for:
"a.m." U+33C2 ㏂ (HTML
"p.m." U+33D8 ㏘ (HTML
㏘) in Unicode.
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.
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", formerly written as 5.5 in some publications in the UK) 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 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. This form is also used in television and radio broadcasts that cover multiple time zones which are at one-hour intervals.
Instead of meaning 5:30, the "half five" expression is sometimes used to mean 4:30, i.e., "half-way to five", especially in the more German-influenced parts of the U.S.A (the Midwest, essentially). The "half-way to five" meaning follows the usage in many Germanic and Slavic languages and in Dutch, Danish, Finnish, Hungarian, Indonesian and Swedish.
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"); 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 pm is conventionally "twenty-eight minutes to seven" rather than "thirty-two minutes past six").
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.
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 strictly speaking "noon" (midday - meridies (m.)) is neither before nor after itself, the terms a.m. and p.m. do not apply. Although "12 m." would be a logically consistent way to indicate noon, this is seldom done (and so risks misunderstanding), and it does not resolve the question of how to indicate midnight.
The American Heritage Dictionary of the English Language has a usage note on this topic: "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."
E. G. Richards in his book Mapping Time provided a diagram in which 12 a.m. means noon and 12 p.m. means midnight. 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.
Many U.S. style guides, and NIST's "Frequently asked questions (FAQ)" web page, 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 explicitly 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." 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 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 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.
The 24-hour clock notation avoids these ambiguities by using 00:00 for midnight at the start of the day and 12:00 for noon and 24:00 for midnight at the end of a day.
In Britain, various conventions are employed. For instance, on 17 December 2005 The Sun (London) newspaper's TV magazine used "noon (12.00)" and "midnight (0.00)" in individual listings. Sequential listings started with a.m. or p.m. as appropriate, but these indicators were not used again, although in sub-listings "12midnight" was sometimes employed. On the same date, London's The Daily Telegraph used "12.00noon" and "12.00midnight" in individual listings. In sequential listings the first programme to start after 12.00 was marked "am" or "pm" as appropriate.