The Excel one seems to have historical justification; that spolsky chap mentions it here: http://www.joelonsoftware.com/items/2006/06/16.html http://www.joelons

The Excel one seems to have historical justification; that spolsky chap mentions it here: http://www.joelonsoftware.com/items/2006/06/16.html http://www.joelonsoftware.com/items/2006/06/16.html. Basically, early Excel was being compatible with Lotus, which ignored 1900 being a leap year probably because of limited memory back then. Possibly MS should have fixed it before Excel 2000, but it's not a simple situation when being correct with leap years breaks compatibility with something else.