Visual FoxPro Calculates Leap Year Correctly in 21st Century

The information in this article applies to:

• Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b

SUMMARY

When a year marking a new century arrives, one would assume that it is a leap year because it can be divided by the number 4. This is not always true for a centesimal year. So, when FoxPro checks to see if a centesimal year is a leap year, there is another condition that must be met.

A year that is a century year must follow another condition to be considered a leap year. If the centesimal year can also be divided by 400 and return an even number, then that year is considered a leap year. For example, the year 1600 is a leap year but 1700, 1800, and 1900 are not. The year 2000 meets the condition, so it will be a leap year.

MORE INFORMATION

Calendars based on the movements of the sun and moon have been used since ancient times, but none have been perfect. The Julian calendar, under which western nations measured time until A.D. 1582 was authorized by Julius Caesar in 46 B.C., the year 709 of Rome. The Julian calendar, on the assumption that the true year was 365 1/4 days, gave every fourth year 366 days. The Venerable Bede, an Anglo-Saxon monk, announced in A.D. 730 that the 365 1/4-day Julian year was 11 minutes, 14 seconds too long, a cumulative error of about a day every 128 years, but nothing was done about it for more than 800 years. By 1582, the accumulated error was estimated to amount to 10 days. In that year Pope Gregory XIII decreed that the day following Oct. 4, 1582, should be called Oct. 15, thus dropping 10 days and initiating what became known as the Gregorian calendar.

However, with common years of 365 days and a 366-day leap year every fourth year, the error in the length of the year would have recurred at the rate of a little more than 3 days every 400 years. Therefore, 3 of every 4 centesimal years (years ending in 00) were made common years, not leap years. Thus, 1600 was a leap year but 1700, 1800, and 1900 were not. The year 2000 will be a leap year. Leap years are those years divisible by 4, except centesimal years, which are common unless divisible by 400.

The year 2000 is arriving soon, and Visual FoxPro is indeed calculating leap year correctly. The following examples will show how to check to see if a centesimal year is a leap year or not.

Type the following in the Command window:

   SET CENTURY ON
   ? {02/28/1900} + 1
   ? {02/28/2000} + 1