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Start and end of day
Midnight marks the beginning and ending of each day in civil time throughout the world. As the dividing point between one day and another, midnight defies easy classification as either part of the preceding day or of the following day. Though there is no global unanimity on the issue, most often midnight is considered the start of a new day and is associated with the hour 00:00. Even in locales with this technical resolution, however, vernacular references to midnight as the end of any given day may be common.
Strictly speaking, it is incorrect to use "a.m." and "p.m." when referring to noon or midnight. The abbreviation a.m. stands for ante meridiem or before noon, and p.m. stands for post meridiem or after noon. Since noon is neither before nor after noon, and midnight is exactly twelve hours before and after noon, neither abbreviation is correct. However, many digital representations of time are configured to require an "a.m." or "p.m." designation, preventing the correct absence of such designators at midnight. In such cases, there is no international standard defining which arbitrary selection is best.In the United States and Canada, digital clocks and computers commonly display 12 a.m at midnight. 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. 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 this change. The US National Institute of Standards and Technology (NIST) recommends avoiding confusion altogether by using "11:59 pm" or "12:01 am" and the intended date instead of "midnight" or "12:00 am".There are several common approaches to identifying and distinguishing the precise start and end of any given day.
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