閏年規則 Leap Year

「4年一閏，百年不閏，四百年閏」

4000年閏

function isLeapYear (year):
if ((year modulo 4 is 0) and (year modulo 100 is not 0)) or (year modulo 400 is 0)
then true
else false

Reference: http://en.wikipedia.org/wiki/Leap_year