Here is a current Wind Chill Warning for Home (Madison, WI) until 12:00pm, Fri., Jan 16, 2009, from your local National Weather Service office. -- Weather Alert EmailSo what time is that, exactly? Noon? Midnight? And for that matter, which midnight -- the one that begins the day, or the one that ends it? Fortunately, from other internal evidence in the weather alert, it's clear that they mean noon, and that it will start warming up a bit Friday afternoon.
The mystery of what 12:00 a.m and p.m actually mean is not so easily resolved. It's a matter of conventions built on top of an oxymoron, and so it's not surprising the conventions vary all over the world and according to which style manual one uses. All the problems go away with a 24-hour clock, but like the metric system, that seems to be too logical for most of us English-speakers, and so we saddle ourselves with these antique Latin a.m. (ante meridiem, before noon) and p.m. (post meridiem, after noon) conventions.
That's what gives rise to that interesting oxymoron -- noon expressed as either 12:00 a.m. or 12:00 p.m. Since noon is by definition 12:00, it can't be either before itself or after itself. Wikipedia has an interesting article on this thorny topic, and they quote the National Maritime Museum, Greenwich.
To avoid confusion, the correct designation for twelve o'clock is 12 noon or 12 midnight. Alternatively, the twenty-four-hour-clock system may be used. The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line). At 12 noon the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither "ante-" nor "post-".As the article and a handy accompanying chart make clear, about the only emerging worldwide standard is that of digital clocks and watches set to the 12-hour display. They all roll over to "p.m." at 12:00 noon. (As the one in the photo did, indicated by the microscopic "P" on the far right above the seconds.)
Amusingly, and not surprisingly, the Feds are no more consistent than anyone else. For the National Weather Service, 12:00 p.m. is noon. For the Government Printing Office, 12:00 p.m. is the following midnight. But at least the Weather Service, high-tech organization that it is, is on the side of the digital convention.