[leap year] うるう年の判別

例１

y=年(整数)

n =  (1 / (y % 4 + 1)) * (1 - 1 / (y % 100 + 1)) + (1 / (y % 400 + 1))

n=1 ==>うるう年

n=0 ==>うるう年

a % bはa - a / b * bで表せる

例２

n =  (1 / (y - y / 4 * 4 + 1)) * (1 - 1 / (y - y / 100 * 100 + 1)) + (1 / (y - y / 400 * 400 + 1))

整数の割り算が割り切れない場合、切り捨てられる

1 / (y % 4 + 1)はyが4の倍数の場合のみ1になる

例3

      LOGICAL FUNCTION LEAPYR(Y)

      INTEGER Y

      LEAPYR=.FALSE.

      IF(MOD(Y,4).NE.0) RETURN

      IF((MOD(Y,100).EQ.0).AND.(MOD(Y,400).NE.0)) RETURN

      LEAPYR=.TRUE.

      END

C====================================

C

C Which years are leap years?

C http://www.timeanddate.com/date/leapyear.html

C

C====================================

C

C In the Gregorian calendar, which is the calendar used by most modern

C countries, the following rules decides which years are leap years:

C

C 1. Every year divisible by 4 is a leap year.

C 2. But every year divisible by 100 is NOT a leap year

C 3. Unless the year is also divisible by 400, then it is still

C a leap year.

C

C This means that year 1800, 1900, 2100, 2200, 2300 and 2500 are **NOT**

C leap years, while year 2000 and 2400 are leap years.

C

C This actually means year 2000 is kind of special, as it is the first

C time the third rule is used in many parts of the world.

C

C In the old Julian Calendar, there was only one rule: Every year

C divisible by 4 is a leap year. This calendar was used before the

C Gregorian calendar was adopted.