12-hour clock
The 12-hour clock has been developed since the second millennium BC and reached its modern form in the 16th century.
The 12-hour time convention is common in several English-speaking nations and former British colonies, as well as a few other countries.
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.
[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 by 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.
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.
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, South Africa, India, Pakistan, and Bangladesh, and others follow this convention as well, such as Mexico and the former American colony of the Philippines.
This behaviour can be changed by the user, such as with the Windows operating system's "Region and Language" settings.
[citation needed] 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.
With the advent of computer generated and printed schedules, especially airlines, advertising, and television promotions, the "M" character is often omitted as providing no additional information as in "9:30A" or "10:00P".
[citation needed] This meaning follows the pattern choices of many Germanic and Slavic languages, including Serbo-Croatian, Dutch, Danish, Russian, and Swedish, as well as Hungarian, Finnish, and the languages of the Baltic States.
These forms are often commonly used in television and radio broadcasts that cover multiple time zones at one-hour intervals.
[23] In describing a vague time of day, a speaker might say the phrase "seven-thirty, eight" to mean sometime around 7:30 or 8:00.
Such phrasing can be misinterpreted for a specific time of day (here 7:38), especially by a listener not expecting an estimation.
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).
[35] Historically, 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.
In the UK, the National Physical Laboratory "FAQ-Time" web page[32] 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 the term at all.
