Leap Year FAQ

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LEAP YEAR FAQ (Frequently Argued Question)

Revision 1.2, 4/4/1998

Q: Is the year 2000 a leap year?
A: Yes, because the rule is: in the Gregorian calendar,

      leap years occur in years exactly divisible by four,
      except that years ending in 00 are leap years
      only if they are divisible by 400.

      So, 1700, 1800, 1900, 2100, and 2200 are not leap years.
      But 1600, 2000, and 2400 are leap years.

Q: But the year 2000 is not a leap year.
A: No, 'tis too, because the rule is: in the Gregorian calendar,

      leap years occur in years exactly divisible by four,
      except that years ending in 00 are leap years
      only if they are divisible by 400.

      So, 1700, 1800, 1900, 2100, and 2200 are not leap years.
      But 1600, 2000, and 2400 are leap years.

Q: But century years aren't leap years.
A: Some of them aren't, but 2000 is, because the rule is:
   in the Gregorian calendar,

      leap years occur in years exactly divisible by four,
      except that years ending in 00 are leap years
      only if they are divisible by 400.

      So, 1700, 1800, 1900, 2100, and 2200 are not leap years.
      But 1600, 2000, and 2400 are leap years.

Q: What makes you think that's the correct rule?
A: Claus Tondering's wonderful Calendar FAQ, v. 1.7,
   http://www.pip.dknet.dk/~pip10160/calendar.html, which says:

        The Gregorian calendar has 97 leap years every 400 years:

        Every year divisible by 4 is a leap year.
        However, every year divisible by 100 is not a leap year.
        However, every year divisible by 400 is a leap year after all.

        So, 1700, 1800, 1900, 2100, and 2200 are not leap years.
        But 1600, 2000, and 2400 are leap years.

Q: What makes you think he knows what's he's talking about?
A: Because he agrees with the 1995 World Almanac, which says:

    "3 out of every 4 centestimal years (years ending in 00) were
    made common years, not leap years.  Thus, 1600 was a leap year;
    1700, 1800, and 1900 were not, but 2000 will be.  Leap years are
    those years divisible by 4, except centesimal years, which are
    common unless divisible by 400."

Q: Why should I believe the 1995 World Almanac?
A: Because it agrees with the American Heritage Dictionary, Third
   Edition, which says: "Leap year: a year in the Gregorian calendar
   having 366 days..." and (under the entry "calendar") "The solar year
   of the Gregorian calendar consists of 365 days, except in a leap year,
   which has 366 days and occurs every fourth, even-numbered year.
   Centenary years are leap years only if they are evenly divisible by 400."

Q: Why do you assume the American Heritage Dictionary is right?
A: Because it agrees with "Field Guide to the Stars and Planets,"
   by Donald H. Menzel (Peterson Field Guide series), which explains
   the Julian leap years and continues "Pope Gregory ... ruled that
   the 'century years,' such as 1900 or 2000, would not contain their
   allotted extra day unless divisible by 400."

Q: The "Field Guide to the Stars and Planets" is not to be trusted.
A: I'd never dream of accepting it without cross-checking in the
   1997 Grolier Multimedia Encyclopedia, which says:

       leap years occur in years exactly divisible by four,
       except that years ending in 00 must be divisible by 400
       to be leap years. Thus, 1600, 1984, and 2000 are leap years,
       but 1800 and 1900 are not.

Q:  Grolier, Grolier, who the hell is Grolier?
A:  Jean Grolier de Servi*res Vicompte d'Aguisy, 1479-1565,
    patron of Aldus Manutius and lover of gold-tooled Moroccan leather
    bookbindings.  But that's not important now.  What's important is
    that Grolier's encyclopedia agrees with the Columbia Encyclopedia
    (at least the version on a shovelware CD called "Our Times"), which says

       In 1582 Pope Gregory XIII ... ordained
       that thereafter the years ending in hundreds should not be
       leap years unless they were divisible by 400. The year 1600
       was a leap year under both systems, but 1700, 1800, and 1900
       were leap years only in the unreformed calendar. The reform
       was accepted, immediately in most Roman Catholic countries,
       more gradually in Protestant countries, and in the Eastern Church
       the Julian calendar was retained into the 20th cent. The
       present generally accepted calendar is therefore called Gregorian,
       though it is only a slight modification of the Julian.

Q: I don't agree with the Columbia Encyclopedia.
A: But THEY pretty much agree with the Encyclopaedia Britannica
   (1997 CD-ROM) which says:

      In the Gregorian calendar now in general use, the discrepancy
      is adjusted by adding the extra day to only those century years
      exactly divisible by 400 (e.g., 1600, 2000).

   Although the then go on to cloud the issue just a bit by adding

      For still more precise reckoning, every year evenly divisible
      by 4,000 (i.e., 16,000, 24,000, etc.) is made a common (not leap)
      year.

    So there could be an enjoyable argument about exactly what the
    Britannica means here.

Q: But none of these are actually _official_.
A: Well, how about the National Institute of Science and Technology?
   They say:

     "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."

     http://www.bldrdoc.gov/timefreq/faq/faq.htm

Q: Bollocks to that. The NIST isn't _my_ National Institute.
   As one of Her Majesty's loyal subjects I bloody well know
   that 1700 was too a leap year in England, so why
   should I believe any of this bumf, you sod?
A: In the Gregorian calendar, 1700 was not a leap year.  Neither
   Great Britain nor her colonies adopted the Gregorian calendar
   until 1752, making it possible to win properly phrased bets
   about the date of George Washington's birthday.  Anyway we're
   all on the Gregorian calendar now, and if you don't like it,
   by jingo we'll kick your sorry butts just like we did in 1812.

Q: But the reason year 2000 is a leap year is that it's divisible
   by 1000.
A: It's true that 2000 IS a leap year, but that is not the
   correct rule. In the Gregorian calendar,

   Century years which are leap years occur every
   four hundred years, not every thousand years.

Q: But what about the 3200-year rule?
Q: But what about the 3600-year rule?
Q: But what about the 4000-year rule?
Q: But what about the years-divisible-by-900-leaving-remainders
   of-200-or-600 rule?

A: As long as you agree that the year 2000 is a leap year, I won't
   give you a hard time.  Everyone understands that the Gregorian
   calendar will be off by about a day in 3000 years or so.  There
   seem to have been various proposals for adding a rule to improve
   the accuracy of the Gregorian calendar, and it is possible that
   some of them may have actually been adopted in some country
   somewhere.

   As far as I can tell, the earliest year about which it is
   possible to have any serious debate is 2800. Hopefully the issue
   will be resolved well before then.

Q: Are you sure that's really what Pope Gregory said?
A: Well, actually, he said:

        Deinde, ne in posterum a XII kalendas aprilis aequinoctium
        recedat, statuimus bissextum quarto quoque anno (uti mos
        est) continuari debere, praeterquam in centesimis annis;
        qui, quamvis bissextiles antea semper fuerint, qualem etiam
        esse volumus annum MDC, post eum tamen qui deinceps
        consequentur centesimi non omnes bissextiles sint, sed in
        quadringentis quibusque annis primi quique tres centesimi
        sine bissexto transigantur, quartus vero quisque centesimus
        bissextilis sit, ita ut annus MDCC, MDCCC, MDCCCC
        bissextiles non sint. Anno vero MM, more consueto dies
        bissextus intercaletur, februario dies XXIX continente,
        idemque ordo intermittendi intercalandique bissextum diem in
        quadringentis quibusque annis perpetuo conservetur.

   I think "annus MDCC, MDCCC, MDCCCC bissextiles non sint" means
   "1700, 1800, and 1900 are not leap years."  And I think
   "Anno vero MM, ... februario dies XXIX continente" means
   "in the year 2000 February will contain 29 days."
More comments on the leap year phenom are now available.