|* See section "Confusion|
at noon and midnight"
The '12-hour clock'is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem (a.m., Latin "before noon") and post meridiem (p.m., "after noon"). Each period consists of 12 hours numbered 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The indication of noon and midnight in the 12-hour system is disputed.
The 12-hour clock was developed over time from the mid-second millennium BC, to the 16th century A.D. and was once popular throughout Northern Europe, but is now used as the dominant system in the United Kingdom and some former British colonies, including India and the United States. It is also used informally in most of the world. The notion has received much criticism, yet survives out of tradition.
The 12-hour clock can be traced back as far as Mesopotamia and Ancient Egypt, but also have roots in Ancient India. The lengths of the ancient hours varied seasonally, always with 12 hours from sunrise to sunset and 12 hours from sunset to sunrise. In Egypt the hour beginning and ending each half-day (four hours each day) were considered twilight hours. An Egyptian sundial for daylight use and an Egyptian water clock for nighttime use found in the tomb of Pharaoh Amenhotep I, 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 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 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 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 the more specialist applications, such as for astronomical clocks and chronometers.
Most analog clocks and watches today use the 12-hour dial, on which the hour hand (shorter and sometimes thicker) rotates once every 12 hours and twice in a day. They 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.
In many European countries, a 12-hour clock is commonly used in informal speech, but a.m. and p.m. are little known. 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.
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, these abbreviations and phrases are often omitted, and one must fall back on one's cultural literacy to disambiguate. 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 is scheduled to begin at "9:00", it means 9:00 p.m.
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. People who grew up with the 12-hour clock often have problems indicating midnight and noon. Those who grew up with the 24-hour clock are confused when they come across situations very common in Internet forums and email, in which a message indicated as posted at "12:46 am" appears before a message marked "11:05 am". (These arguments may be compared to the discussion on metrication.)
The disadvantages commonly voiced in comparing the 12-hour notation to the 24-hour clock are:
(start of day)
(end of day)
|24-hour clock, ISO 8601||00:00||12:00||24:00|
|Most digital 24-hour clocks||00:00||12:00||—|
|12-hour digital clocks with a.m. and p.m. *||12:00 a.m.||12:00 p.m.||—|
|U.S. Government Printing Office||—||12 a.m.||12 p.m.|
|Antiquated †||12:00 m.n.||12:00 m.||12:00 m.n.|
|NIST2 †||12:00 Midnight||12:00 Noon||12:00 Midnight|
|Associated Press Style||midnight||noon||does not exist|
|U.S. de facto legal||12:01 a.m.||—||11:59 p.m.|
|Encyclopædia Britannica|| Midnight|
|* Digital clocks and computers appear to show the times 12 a.m. and 12 p.m., as in this chart. While those phrases may be used practically, the "a.m." and "p.m." refer to the 12-hour periods following the instants of midnight and noon, respectively, not to the instants of midnight and noon themselves, strictly speaking.|
|† These standards are ambiguous with respect to the whether midnight is at the start and or end of each day. The style guide writers did not state their intention for the two midnights or for endpoint convention.|
The National Maritime Museum, 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-".
In the United States, noon is often called "12:00 p.m." and midnight "12:00 a.m.". With this convention, thinking of "12" as "0" makes the system completely logical.
The American Heritage Dictionary of the English Language (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.
The use of "12:00 a.m." for midnight and "12:00 p.m." for noon, however, is contrary to the U.S. Government Printing Office Style Manual which recommends the opposite: "12 p.m." for midnight and "12 a.m." (formerly "12 m.") for noon.
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, bus, or train schedules, though some schedules use other conventions.
The 24-hour clock 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. Some variants of 24-hour notation (including the world standard ISO 8601) use 00 and 24:00 when referring to a midnight at the end of a day.
Which endpoint convention to use depends on how one regards time. If time is regarded as discrete then a right or left end point convention should be used; left is preferable when the value has to be non-negative as it places zero with the positive numbers. If time is regarded as continuous and calculus is to be performed on functions of time then a closed interval should be used.
The use of a.m. as written in the form of am, AM, or A.M. can be confusing because am is an English word, AM is an abbreviation for amplitude modulation and A.M. is an abbreviation for anno mundi, in the year of the world, and for Master of Arts.
There are symbols for "a.m." (U+33C2 = "㏂") and "p.m." (U+33D8 = "㏘") in Unicode. They are meant to be used only with CJK fonts, however, as they take up exactly the same space as one Chinese character.
Stylebooks use a space between the number and the abbreviation a.m. or p.m., although that convention is widely violated.
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. Generic words like "evening", however, do not apply for the whole year.
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 of the U.S.A (the Midwest, essentially). "Half-way to five" follows the usage in German speaking countries. It is also found in Danish, Swedish, Dutch, Lithuanian and Russian.
In the UK 5:25 or 5:35 can sometimes be referred to as "five and twenty past five", and "five and twenty to six" respectively, but this usage is quite rare and generally restricted to the older generation, with most people opting to use "five twenty-five" or "twenty-five past five", and "twenty-five to five" respectively.
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 (aught one to aught 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.
When the speaker has recently mentioned the hour of the day or for some other reason believes it to be known to his or her hearers, he or she may omit all reference to it and simply declare the minutes, using expressions such as seventeen minutes past the (top of the) hour (to refer to 4:17 am, or 11:17 pm, etc.) or three minutes till the bottom of the hour (which similarly signals the bottom half of the clock, such as 7:27 pm, or 9:27 am, etc.). This is also true of broadcasts whose signals are picked up in more than one time zone, since the hour varies with those zones.