Share via Symptoms Microsoft Excel incorrectly assumes that the year 1900 is a leap year. This article explains why the year 1900 is treated as a leap year, and outlines the behaviors that may occur if this specific issue is corrected. More Information When Lotus 1-2-3 was first released, the program assumed that the year 1900 was a leap year, even though it actually was not a leap year. This made it easier for the program to handle leap years and caused no harm to almost all date calculations i...