May 24, 1994
(Prepared by NIST's Time and Frequency Division, Boulder, Colo.)
Additional Q&A on Time

Time Questions and Answers from NIST

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

This is perhaps the trickiest time question of them all. The best answer is that the terms 12 a.m. and 12 p.m. cause confusion 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. Of course, noon is neither before nor 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.

To get around the problem, the terms 12 noon and 12 midnight should be used instead of 12 a.m. and 12 p.m. For example, a bank might be open on Saturday from 8 a.m. to noon. Or, a grocery store might be open daily until midnight. If you are making schedules, times such as 12:01 a.m. (one minute after midnight), or 11:59 p.m. (one minute before midnight) also can eliminate ambiguity. This method is used by the railroads and airlines.

Is Coordinated Universal Time or UTC the same thing as Greenwich Mean Time?

Greenwich Mean Time is a 24-hour astronomical time system based on the local time at Greenwich, England. GMT can be considered equivalent to Coordinated Universal Time (known as UTC) when fractions of a second are not important. However, by international agreement, the term UTC is recommended for all general timekeeping applications and use of the term GMT is discouraged.

How is the second defined?

The international definition of a second is "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom." This definition was agreed upon in 1967. Atomic clocks based on the cesium atom then became the primary means for accurate timekeeping.

What is a leap second?

A leap second is a second added to Coordinated Universal Time to make it agree with astronomical time to within 0.9 seconds. UTC is an atomic time scale, based on the performance of atomic clocks. Astronomical time is based on the rate of rotation of the Earth. Since atomic clocks are more stable than the rate at which the Earth rotates, leap seconds are needed to keep the two time scales in agreement.

The first leap second occurred on June 30, 1972. There have been a total of 18 leap seconds to this date. This means that leap seconds occur at a rate of slightly less than one per year. Although it is possible to have a negative leap second (a second removed from UTC), so far, all leap seconds have been positive (a second has been added to UTC). Based on what we know about the Earth's rotation, it is unlikely that we will have a negative leap second in the foreseeable future.

Is the year 2000 a leap year?

The year 2000 will be a leap year. Century years (like 1900 and 2000) are only considered leap years if they are evenly divisible by 400. Therefore, 1700, 1800 and 1900 were not leap years, but the year 2000 will be a leap year.

To understand this, you need to know why leap years are necessary in the first place. Leap years are necessary because the actual length of a year is 365.242 days, not 365 days, as commonly stated. Therefore, on years that are evenly divisible by four (like 1992, for example) an extra day is added to the calendar on Feb. 29. However, since the year is slightly less than 365.25 days long, adding an extra day every four years results in about three extra days being added over a period of 400 years. For this reason, only one out of every four century years is considered as a leap year.

What is an atomic clock?

An atomic clock is a clock that keeps time using natural characteristic frequencies of atoms, such as cesium, hydrogen or rubidium. Atomic clocks are extremely stable because the atom's characteristic frequencies are not affected by factors like temperature, pressure or humidity.

Who regulates time zones? What is their history?

Time zones are regulated by the U.S. Department of Transportation and not NIST as commonly believed. Time zones originally were controlled by the Interstate Commerce Commission because the need for time zones came about when railroads were first used for interstate commerce. The United States was first divided into four time zones (Eastern, Central, Mountain and Pacific) on Nov. 18, 1883. In 1967, a congressional act transferred the duties of the ICC to the DOT. It is interesting to note that the DOT (and not NIST) is also responsible for the rules governing Daylight Saving Time.

How long are a nanosecond, a picosecond or a femtosecond?

A nanosecond is one billionth of a second, and a picosecond is one trillionth of a second. Timekeeping technology has not yet reached the stage where we can measure femtoseconds. However, just for the record, a femtosecond is a thousand times smaller than a picosecond!

On what date will the 21st century begin?

This is a date that no organization, including NIST, has the authority to regulate. However, one logical answer to the question is that because there was never a year "zero," and a century must have 100 years, then each century must begin with a year numbered "1." In other words, the 20th century should be considered as ending on Dec. 31, 2000, and the 21st century as starting on Jan. 1, 2001.

However, human nature being what it is, most of us will still opt to have that "once-in-a-century" New Year's Eve bash on Dec. 31, 1999.

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