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`.

## Useful links

This page last updated January 9, 2007.