• Are noon and midnight 12 a.m. or 12 p.m.?
• Why is UTC used as the acronym for Coordinated Universal Time instead of CUT?
• Does the switch to daylight saving time affect UTC?
• When did the current millennium begin?
• What is a Modified Julian Date (MJD)?

Are noon and midnight 12 a.m. or 12 p.m.?

This is a tricky question. The answer is that the terms 12 a.m. and 12 p.m. are wrong and should not be used.

To illustrate this, consider that "a.m" and "p.m." are abbreviations for "ante meridiem" and "post meridiem." They mean "before noon" and "after noon," respectively. Noon is neither before or after noon; it is simply noon. Therefore, neither the "a.m." nor "p.m." designation is correct. On the other hand, midnight is both 12 hours before noon and 12 hours after noon. Therefore, either 12 a.m. or 12 p.m. could work as a designation for midnight, but both would be ambiguous as to the date intended.

When a specific date is important, and when we can use a 24-hour clock, we prefer to designate that moment not as 1200 midnight, but rather as 0000 if we are referring to the beginning of a given day (or date), or 2400 if we are designating the end of a given day (or date).

To be certain of avoiding ambiguity (while still using a 12-hour clock), specify an event as beginning at 1201 a.m. or ending at 1159 p.m., for example; this method is used by the railroads and airlines for schedules, and is often found on legal papers such as contracts and insurance policies.

If one is referring not to a specific date, but rather to several days, or days in general, use the terms noon and midnight instead of 12 a.m. and 12 p.m. For example, a bank might be open on Saturdays from 8 a.m. to noon. Or a grocery store might be open daily until midnight. The terms "12 noon" and "12 midnight" are also correct, though redundant.

Why is UTC used as the acronym for Coordinated Universal Time instead of CUT?

In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT, or the French word order, TUC, the acronym UTC was chosen as a compromise.

Does the switch to daylight saving time affect UTC?

No. UTC, which refers to time on the zero or Greenwich meridian, is not adjusted to reflect changes either to or from Daylight Saving Time. Universal Time (UT) is the general designation of time scales based on the rotation of the Earth.

For additional information, please see an exhibit on daylight saving time.

When did the current millennium begin?

Since a millennium is 1000 years, and the first millennium began at the start of the year 1, it ended at the end of the year 1000. The second millennium then began with the year 1001 and concluded at the end of the year 2000. Therefore, the current millennium technically began with the year 2001. For historical information, see an exhibit on Calendars.

What is a Modified Julian Date (MJD)?

This is a continuous count of the number of days elapsed since 17 November 1858. It is often more useful than conventional calendar dates for record keeping over long periods of time, since the MJD's of two events can easily be subtracted to determine the time difference in days. Usually, the MJD is specified as a number with 5 significant digits. As an example, the MJD for 1 January 1995 is 49718, meaning that this many days have elapsed between 17 November 1858 and 1 January 1995.