Time Questions and Answers from NIST
     (Prepared by NIST's Time and Frequency Division, Boulder, Colo.)


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.