Julian Period

  Astronomers use the Julian period because it is convenient
  to express long time intervals in days rather than months,
  weeks and years.  It was devised by Joseph Scaliger, in
  1582, who named it after his father Julius, thus creating
  the confusion between the Julian (Caesar) calendar and the
  Julian (Scaliger) period.

  Julian Day 1 began at 12:00 noon, January 1, 4713 B.C. This
  date was thought by some to correspond approximately to the
  beginning of the universe.  Certainly it predated any known
  astronomical events known in the 16th century without
  resorting to negative times.  Scaliger decided on the actual
  date on the grounds that it was the most recent coincidence
  of three major chronological cycles:

  -   The 28-year solar cycle, after which dates in the Julian
      calendar (for example September 27) return to the same
      days of the week (for example Tuesday).

  -   The 19-year lunar cycle, after which phases of the moon
      return to the same dates of the year.

  -   The 15-year indiction cycle, used in ancient Rome for tax
      regulation.

  It takes 7980 years to complete the cycle.  Noon of January 1,

  1988, marks the beginning of Julian Day 2447161.

  The Julian period is also of interest because of its use as a
  time base by the OpenVMS operating system.

  Additional topics:

  *   OpenVMS and the Julian Period

  *   Tru64 UNIX Time Origin

  *   Brief History of the Gregorian Calendar

  *   History and DECwindows Calendar