# Was 2000 a leap year?

Is (year % 4 == 0) an accurate test for leap years? (Was 2000 a leap year?)

No, it's not accurate (and yes, 2000 was a leap year). The actual rules for
the present Gregorian calendar are that leap years occur every four years, but not
every 100 years, except that they do occur every 400 years, after all. In C, these
rules can be expressed as:

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

Actually, if the domain of interest is limited (perhaps by the range of a time_t)
such that the only century year it encompasses is 2000, the expression

(year % 4 == 0) /* 1901-2099 only */

is accurate, if less than robust.

If you trust the implementor of the C library, you can use mktime to determine whether
a given year is a leap year;

Note also that the transition from the Julian to the Gregorian calendar involved
deleting several days to make up for accumulated errors. (The transition was first
made in Catholic countries under Pope Gregory XIII in October, 1582, and involved
deleting 10 days. In the British Empire, eleven days were deleted when the Gregorian
calendar was adopted in September 1752. A few countries didn't switch until
the 20th century.) Calendar code which has to work for historical dates must therefore
be especially careful.

Posted by:Richards
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