From...

March 5, 1999
Web posted at: 12:31 p.m. EST (1731 GMT)

by Mitch Betts

(IDG) -- The reason there continues to be confusion about whether the year 2000 is a leap year -- and it definitely is -- is that there are three rules for determining a leap year.

Every school kid is told that leap years occur every four years, but there is an exception and an exception to the exception. Leap years occur every four years except for century years (such as 1900) but including every 400th year (such as 2000). In other words, 1900 wasn't a leap year, 2000 will be and 2100 won't be.

The problem is that some computer programs implement only the four-year rule

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and the 100-year rule but fail to recognize the year 2000 as a leap year via the third rule.

Julius Caesar created the four-year rule to make up for the fact that one-quarter of a day is left over at the end of each year. So an extra day was created every four years, thereby expanding the Julian calendar from 365 to 366 days. The extra day is Feb. 29.

However, the length of the year isn't exactly 365.25 days, but closer to 365.2422. By the 16th century, the resulting round-off error had accumulated to the point where the Roman Catholic Church became seriously concerned because religious holidays weren't being celebrated on the right days. The calendar was off by 10 days.

So Pope Gregory XIII created two new leap year rules: the 100-year rule that excludes century years and the 400-year rule that re-includes them.

As a onetime correction, to get things back in sync, Thursday, Oct. 4, 1582, was followed by Friday, Oct. 15, 1582. Now it's called the Gregorian calendar.