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The 12-hour clock is a time-keeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (am, Latin for "before noon") and post meridiem (pm, Latin for "after noon"). Each period consists of 12 hours numbered 12, 1, 2, ..., 11.1 Usage
Because the numbering of hours starts at 12, 12:00 am is properly the midnight at the start of the day, and 12:00 pm is noon. However, the ordering of the hours means that many people, even many of those who use the 12-hour clock regularly, are confused as to which of 12:00 am and 12:00 pm is which. Therefore, for clarity, it is often best to write "noon" instead of "12:00 pm". Using this clock, there is no unambigious way to write "12:00 am", as "midnight" can mean either the midnight at the start of the day or the midnight at the end. Some style policies specify or suggest "12:00 N" for noon and "12:00 M" for midnight.
Because of the confusion possible with midnight, some legal contracts start or end at 12:01 am, which removes the uncertainty.
2 Typography
Style policies typically frown on use of a preceding zero in the hour when the time of day is expressed in a 12-hour clock format, for example "3:52 pm" is preferred over "03:52 pm" (which may tend to confuse some people trained to use the 24-hour clock), although the default modes of many computer applications and time reporting devices fail to respect this convention.
3 History
Historically the 12-hour clock originated with the Sumerians, although their clock varied seasonally, as it had 12 hours from dusk to dawn, and 12 hours from dawn to dusk.
4 See also
Timekeeping
