IN MS EXCEL 1900 IS A LEAP YEAR I was writing a Python program to find the exact date of Easter Sunday for a given year after 1900 as an example for the course in Discrete Mathematics which I am teaching to 2nd year undergraduate students in my university. A part of the program calls for computing the number of days from 1900 to the given year. Hence, I have to consider the number of leap years. To derive the formula for finding the number of days, I used MS EXCEL since it has an excellent resource about DATES. However, I found out that MS EXCEL accepts the string '2/29/1900' as a legal date and even considers '2/29/1900' as a WEDNESDAY. Also, MS EXCEL recognizes January 1, 1900 as a SUNDAY. To verify these MS EXCEL results, I searched the WEB for the topic on PERPETUAL CALENDAR which showed that JANUARY 1, 1900 is a MONDAY and not a SUNDAY. Furthermore, I found out that a LEAP YEAR is defined as a YEAR which satisfies at least one of the two conditions: 1. The YEAR is a multiple of 4 and not a multiple of 100. 2. If the YEAR is a multiple of 100 then it must be a multiple of 400. Hence, 1996 and 2004 are leap years since they satisfy the first condition. Also, 1600 and 2000 are leap years since they satisfy the second condition. However, 1900 is NOT a LEAP YEAR since it is a multiple of 4 and a multiple of 100 but not a multiple of 400. The error in MS EXCEL is only confined to the dates from January 1, 1900 to February 28, 1900 (an ASH Wednesday) and the illegal date of February 29, 1990. Thus, a mistake occurs in the computation if these dates are involved. An example is the problem of finding the number of days between January 1, 1900 and December 31, 2005. It assigns a serial number of 1 to January 1, 1900 and a serial number of 39,448 to January 1, 2008. However, the correct serial number to January 1, 2008 should be 39,447. The error is due to the existence of February 29, 1900 (serial number of 60) as a valid date in MS EXCEL. Fortunately, I discovered this problem and I was successful in writing a Python program that returns the exact date of Easter Sunday for a given year after 1900. -- Felix P. Muga II, Ph.D. Associate Professor Mathematics Department Ateneo de Manila University