There's a difference between what dates mean to you and what they mean to Excel. To you, 8/22/2010 is a month, day, and year. To Excel, 8/22/2010 is serial number 40412. In this lesson you'll learn how Excel's view of dates makes it possible to do date arithmetic, and how to enter dates in a format that Excel can understand. Learning a little about dates before you do the math will save you time later on.