Microsoft Excel

Microsoft Excel is a spreadsheet editor developed by Microsoft for Windows, macOS, Android, iOS and iPadOS. It features calculation or computation capabilities, graphing tools, pivot tables, and a macro programming language called Visual Basic for Applications (VBA). Excel forms part of the Microsoft 365 and Microsoft Office suites of software and has been developed since 1985.

Excel considers February 29, 1900 to be a valid date, incorrectly treating 1900 as a leap year, even though the Gregorian calendar excludes years divisible by 100 from being leap years unless they are also divisible by 400. Thus, a formula counting dates between (for example) February 1, 1900 and March 1, 1900 will return an incorrect result. The bug originated from Lotus 1-2-3 where it likely originated from the developers carelessly assuming they understood the Gregorian calendar rather than checking the standard.

This legacy has later been carried over into Office Open XML file format.