Leap Year bug.

So your computer survived Y2K. Well, get ready for the next millennium bug: the Leap Year Bug! February 29, 2000 will actually be an extra leap day--the first in 400 years. Could your PC take one flying leap and crash?

Leap years have an extra day at the end of February and occur every four years--1988, 1992, 1996. But there's an exception to this rule: At the start of each century, Gregorian calendars, which are now used worldwide, ignore leap years, and February has only 28 days--there was no leap day in 1700, 1800, or 1900. However, every 400 years the calendar includes the leap day. So there was a February 29 in 1200, 1600, and now 2000. "February 29, 2000 is the exception to the exception," says Rick Weirich, U.S. Postal Service vice president for information technology.

Why are there leap years anyway? Ancient Roman mathematicians and astronomers observed that Earth's orbit around the sun takes slightly longer than 365 days (it's actually 365.2422 days). To make sure the calendar wouldn't go awry, in 46 B.C. Emperor Julius Caesar decided to add one extra day to the calendar every four years.

But after 1,600 years, calendars lagged 10 days behind Earth's revolution around the sun. So in 1582, Pope Gregory skipped 10 days in the calendar--after September 4 came September 15! Then he added a new leap-year rule: Every year ending in 00 will have no leap day, unless it is evenly divided by 400. With this adjustment, calendars have an average of 365.2425 days a year--it's almost perfect!

Now you know the rules, but many computer programmers might have overlooked the exception to the exception in their software programs. "in some place somewhere there will be computer errors," predicts Weirich, "but most computers are prepared for this unique day." Is your PC leap-day 2000 compatible?