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The American Heritage Dictionary of the English Language states "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". [34] E. G. Richards in his book Mapping Time (1999) provided a diagram in which 12 a.m. means noon and 12 p.m. means midnight. [35]
In the 24-hour time notation, the day begins at midnight, 00:00 or 0:00, and the last minute of the day begins at 23:59. Where convenient, the notation 24:00 may also be used to refer to midnight at the end of a given date [3] — that is, 24:00 of one day is the same time as 00:00 of the following day.
Date and time notation in the United Kingdom records the date using the day–month–year format (31 December 1999, 31/12/99 or 31/12/1999). The time can be written using either the 24-hour clock (23:59) or the 12-hour clock (11:59 p.m.), either with a colon or a full stop (11.59 p.m.).
In common usage, a day starts at midnight, written as 00:00 or 12:00 am in 24-or 12-hour clocks, respectively. Because the time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time .
The 30th edition of the U.S. Government Style Manual (2008), in sections 9.54 and 12.9b, recommended the use of "12 a.m." for midnight and "12 p.m." for noon. [3] [4] However, the previous 29th edition of the U.S. Government Printing Office Style Manual (2000), in section 12.9, recommended the opposite. There is no further record documenting ...
In these cases, exact and unambiguous communication of time is critical. If someone mistakes 5:00 AM for 5:00 PM in a hospital for example, when medication or other medical treatment is needed at a certain time, the outcome could be dire. Thus 24-hour time (5:00 PM written as 17:00) is used.
The English term noon is also derived from the ninth hour. This was a period of prayer initially held at three in the afternoon but eventually moved back to midday for unknown reasons. [12] The change of meaning was complete by around 1300. [13] The terms a.m. and p.m. are still used in the 12-hour clock, as opposed to the 24-hour clock.
In 12-hour striking, used most commonly in striking clocks today, the clock strikes once at 1:00 am, twice at 2:00 am, continuing in this way up to twelve times at 12:00 mid-day, then starts again, striking once at 1:00 pm, twice at 2:00 pm, and the pattern continues up to twelve times at 12:00 midnight.