OK don't know what you know so am just going to try and fully explain. I am no expert though as you can see from the lack of stars!
When a date is typed into Excel, it is converted to a number and then the number in the cell is formatted to a Date format. You example, if the default date format is mm/dd/yy (the default is set by the Regional Settings in The Control Panel), when you type in 1/1/03, the cell value is converted to 37622 and it is formatted to 01/01/03. The cell value remains 37622 and can be viewed by changing the cell format property (e.g. to General).
The Date system default in Excel is the 1900 system, so 1/1/1900 is represented by the number 1. Then it increases by 1 for each day. So today is 09/27/03 (I'm in England!) so it is 37891, divide that by 365 and you get approx 103 is how many years has roughly passed.
If times are entered, they are represented as a decimal.
Date cells therefore are numbers and not strings, which can be pain sometimes so used precede the entry with ' to force the text property.
Great inquisitiveness. Its always good to understand how things work rather than just using them, which this site allows rather well.
Regards
jimmid