| Sources | 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."[9]
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[10] 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 n" for noon and "12:00 m" for midnight,[11] but that conflicts with the older tradition of using "12:00 m" for noon[1](Latin meridies), and "12:00 mn" 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 24:00 when referring to a midnight at the end of a day.
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