(someone has to document it with screenshots of Excell 2007)
Quote from The Chernobyl Design Pattern:
type =WEEKDAY("1/1/1900") into Excel. What do you get? It returns 1, meaning Sunday. Now look at any reputable calendar created since Pope Gregory XIII. What day of the week was January 1st, 1900? The correct answer is Monday. So yes, Excel has a bug, and yes Microsoft has pushed to include this bug in an International Standard. To say this is not a bug is pure denial.