Home > 12-hour clock

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

• 24-hour clock