The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods:^[1] a.m. (from the Latin ante meridiem, meaning "before midday") and p.m. (post meridiem, "after midday").^[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 12-hour clock was developed over time from the mid-second millennium BC to the 16th century AD.

History and use [link]

Exeter Cathedral clock, showing the Double-XII numbering scheme

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 nighttime 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 (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".^{[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 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. The 12-hour clock was used throughout the British empire but has largely been replaced by the 24-hour clock.

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 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. The 12-hour clock can be found in countries where the 24-hour clock is preferred.^{[citation needed]}

Use by country [link]

A typical analog 12-hour clock

• The 12-hour clock is the dominant system of time written and spoken in:
  • Australia
  • Bangladesh
  • English-speaking Canada
  • Colombia
  • Costa Rica
  • Egypt
  • El Salvador
  • Ghana
  • Honduras
  • Hong Kong
  • India
  • Iran
  • Ireland
  • Jordan
  • Malaysia
  • Mexico
  • New Zealand
  • Nicaragua
  • Nigeria
  • Pakistan
  • Romania
  • Philippines
  • Saudi Arabia
  • Singapore
  • Taiwan
  • United Kingdom
  • United States
  • Venezuela

In most other countries the 12-hour clock is used in speech alongside the 24-hour clock.^{[citation needed]}

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.^{[citation needed]} 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)^[7] uses them, and so does a similar almanac for 1773.

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

Abbreviations [link]

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.^[8] The equivalents in Greek 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.^{[citation needed]}

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 is scheduled to begin at "9:00", it means 9:00 p.m.

Related conventions [link]

Typography [link]

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 "AM" and "PM" are variously written in small capitals ("am" and "pm"), 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. 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.

Some stylebooks 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,^[9] 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 [link]

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^[10]) 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".^[11] 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 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, Norwegian, Danish, Swedish, Dutch, Serbian, Estonian, Latvian, Lithuanian, Polish, Czech, Slovak, Finnish, Hungarian, Russian and Afrikaans.

Formal speech and times to the minute [link]

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.

U.S. military speech and writing [link]

See Military time

Confusion at noon and midnight [link]

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) is neither before nor after itself, the terms a.m. and p.m. do not apply. However, since 12:01 p.m. is after noon, it is common to extend this usage for 12:00 p.m. to denote noon. That leaves 12:00 a.m. to be used for midnight at the beginning of the day, continuing to 12.01 a.m. that same day.

The American Heritage Dictionary of the English Language (Fourth Edition, 2000) has a 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."^[15]

Many U.S. style guides, and NIST's "Frequently asked questions (FAQ)" web page,^[16] 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."). Some other style guides suggest "12:00 n" for noon and "12:00 m" for midnight.^[17]

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 these 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".^{[citation needed]}

See also [link]

• 24-hour clock
• Clock position
• Date and time representation by country
• Decimal time
• Midnight
• Noon
• Thai six-hour clock

References [link]

External links [link]

• NIST FAQ on time
• 12am is noon in Japan