I saw a formula somewhere but I can't find it anymore, so
 if anyone can send it to me.

 Thanks in advance.

 Mark.

--

It is a leap year.

Formula :

Divisable by 4,  not divisable by 100 or also divisable by 400

Oh, NO! Not this discussion again please: 2000 IS a leap year (Centuries are
leap years when divisible in 400)

      - if a year is divisible by 100 then it must be also be divisible by 400 to be a
        leap year

1) If the year number is dividable (without reminders) with 4, that year is
a leap year.
2) If the year number is dividable with 100 (i.e. 'ends with 2 zero's'), that
year is NOT a leap year (although it's, of course, dividable with 4).
3) If the year number is dividable with 400 (which is case of the year 2000),
that year IS a leap year (although it's still dividable with 100).

There have been too many leap years and because there are seconds of
difference on the leap years they have to take out a leap year in order
to keep the time accurate.

"According to the Gregorian rule of intercalculation, therefore, every year
of which the the number is divisible by four is a leap year, excepting the
centurial years, which are only leap years when divisible by four after
omitting the two ciphers.  Thus 1600 was a leap year, but 1700, 1800 and
1900 are common years; 2000 will be a leap year, and so on."

A 'normal' year has 365 full days. After that time the earth is supposed to have
travelled full circle around the sun. As with many suppositions, this is not completely
true, in fact it takes the earth 365.2422 days to circle the sun. So after one 'normal'
year it is 0.2422 days behind.

After 4 years this accumulates to 1 day, and therefore a leapday is inserted.
But this over-corrects as this makes the year 365.25 days long.

After 100 years this accumulates to 1 day of over-correction, so the leapday is omitted
if the year is divisible by 100. This correction makes the year 365.24 days long.

After 400 years, this accumulates to 1 day of over-over-correction (are your still
there?), so a leapday is inserted if the year is divisible by 400. This makes the year
365.2425 days.

As you can see the year is now 0.0003 days too long, each 3333.333 years the leapday
should be dropped again. I've never seen a correction factor for this difference.

This method of determining leapyears was decreed by Pope Gregory XIII in 1582 and is
therefore called the Gregorian Calendar. It replaced the Julian Calendar which was
authorized by Julius Ceasar in 46 BC.

The Julian Calendar had a leapday every 4 years so after 1600 years the seasons and the
calendar where out of phase by 11 days. To compensate this, October 4 1582 (Julian Date)
was followed by October 15 1582 (Gregorian Date).

The following WEB-site has more information about time-related stuff:

   http://www.ast.cam.ac.uk/RGO/leaflets/

:> I think that it would significantly reduce the number and lengths of the
:> threads on the topic over the next four years if we could locate a truly
:> authoritative source available as a URL on a good WWW site - and who

:"According to the Gregorian rule of intercalculation, therefore, every year
:of which the the number is divisible by four is a leap year, excepting the
:centurial years, which are only leap years when divisible by four after
:omitting the two ciphers.  Thus 1600 was a leap year, but 1700, 1800 and
:1900 are common years; 2000 will be a leap year, and so on."

Let's see everyone please if I managed to get it right for the next
version of our FAQ as John suggested. Here goes with a one-line
boolean evaluation.

function ISLEAP (y : integer) : boolean;
begin
  isleap := (y mod 4 = 0) and not ((y mod 100 = 0) and not (y mod 400 = 0));
end;  (* isleap *)

At least it passes the above quote by Tom, and gives the same
results as was suggested earlier using a longer solution of nested
if statements by JSha...@nmsu.edu.

   All the best, Timo

[...]
From: s...@eskimo.com (Steve Summit)
Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List)
[...]

20.32:  Will 2000 be a leap year?  Is (year % 4 == 0) an accurate test
        for leap years?

A:      Yes and no, respectively.  The full expression for the present
        Gregorian calendar is

                year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)

        See a good astronomical almanac or other reference for details.
        (To forestall an eternal debate: references which claim the
        existence of a 4000-year rule are wrong.)