Leap day

# Leap day

The leap day is a day added to the year on February 29, to adjust for the drift of the seasons in the Julian calendar. Eric Weisstein's World of Astronomy (http://scienceworld.wolfram.com/astronomy/) has a good graphical illustration of how this correction works for the Gregorian calendar. The Julian calendar simply added the leap days if YEAR mod 4 = 0.

The Gregorian calendar modified this so that a year was a leap year if YEAR mod 4 = 0 except if YEAR mod 100 = 0 unless YEAR mod 400 = 0. This is the calendar in use in most of the world today.

The Russian calendar (also called the "Revised Julian Calendar"), however, is slightly more accurate, if not quite as easy to remember. The first two parts of the rule (MOD 4 and MOD 100) are the same, but the third part (MOD 400) is replaced by YEAR mod 900 = 200 or 600.

Practially, the results are going to be the same between the Gregorian and Russian calendars for many centuries. The first year the two calendars will disagree is the year 2800 which will have a leap day added in the Gregorian but not the Russian calendar. Assuming the two calendars are both still in existance, the discrepancy will last only one century until 2900 which will have a leap day in the Russian but not the Gregorian calendar. The calendars will then agree until 3200.

Note: "x MOD y" means the remainder when x is divided by y. So if 11 / 4 is 2 with a remainder of 3, 11 MOD 4 is 3.