leap year

One should note that a year is a leap year if it is divisible by 4. Unless it is divisible by 100; however, if it is divisible by 400 it is a leap year. There are no further rules.

For more information: <http://aa.usno.navy.mil/AA/faq/docs/leap_years.html>

An alternative system that is conceptually simpler, very elegant in binary and very accurate is the following:

• Each year is not a leap year...
• ...unless it is divisible by 4, in which case it is...
• ...unless it is divisible by 128, in which case it is not.

This has the advantage of being faster to evaluate on a binary computer, and only involves two tests rather than the current three. The disadvantage is that no-one uses it, so if you do, you'll be wrong, and it's harder to do mental division by 128 in decimal than by 400. Non-optimal C implementation below.

int is_alt_leap (int year) {
   int leap = 0;
   if ((year & 0x03) == 0) {
      leap = 1;
      if ((year & 0x7F) == 0)
         leap = 0;
   }
   return leap;
}

This gives 354 days a year, instead of the ~365.24 you need to keep in sync with the solar calendar. So 7 out of 19 years get an extra month -- Adar becomes Adar 1 followed by Adar 2.

The rule for determining if a year is a leap year is simple: just determine the year (counting from epochial Creation, 5761 years ago) modulo 19. If the remainder is 0, 3, 6, 8, 11, 14 or 17 then it's a leap year.

This gives a solar calendar slightly more accurate than the Gregorian calendar.

Moral:

Leap" year` (?)

. Bissextile; a year containing 366 days; every fourth year which leaps over a day more than a common year, giving to February twenty-nine days. See Bissextile.

⇒ Every year whose number is divisible by four without a remainder is a leap year, excepting the full centuries, which, to be leap years, must be divisible by 400 without a remainder. If not so divisible they are common years. 1900, therefore, is not a leap year.

© Webster 1913.