Entering Dates and Times
Although Excel records dates and times as serial date values, you don’t have to type them that way. You can manipulate dates and times in your worksheet formulas just as you manipulate other types of values. You enter date values in formats that Excel automatically applies. To enter date values in this way, type the date in one of the following formats: m/d/yy, d-mmm-yy, d-mmm, or mmm-yy. (You can also type four-digit years for any of these formats.)
You can change the default date, time, currency, and numbering settings through the Clock, Language, And Region item in Windows Vista Control Panel (Date, Time, Language, And Regional Options in Windows XP). These settings will determine how Excel interprets your date entries. ...