Dates are a special case in Excel. If you enter information that can be translated as a date (by any stretch of the imagination), then Excel treats it as a date. It converts your data into a serial number that is internally used to represent dates and times. For instance, any of the following entries will be translated to a date by Excel:
- 15 Dec
- December 15, 2012
If you enter the first example, Excel will convert it to a date and display that date as best it can determine. If you leave off the year in your entry, Excel assumes you mean the current year. You could also use dashes instead of slashes and Excel will still figure out you are entering a date.
Regardless of how you type a date, it is converted to a special serial number by Excel and stored internally in that format. How you see the date on your screen is a consequence of how the cell is formatted. Even though Excel stores dates in a standard format internally, they can be displayed using any number of different formats.