In Excel for Windows, a date is a serial number, where 1=Jan 1, 1900.

2=Jan 2, 1900; 29=Jan 29, 1900; 8/10/2011=40765, etc.

This is the reason you can do math with dates and the answer will be in number of days. Try it... put your next birthday in a A1. Put today's date in A2. Create a formula =A1-A2. the answer is how many days til your next birthday. (If the answer looks like a date, just change the format to number.)

No matter how you type a date in a cell, no matter how you format it, the formula bar will show it as 1/5/2008. If it doesn't, then Excel doesn't recognize that it is a date, and that is a big big problem.

Type in the date however you find easiest to type and then format it the way you like to see it.

By the way, times are just fractions of dates... Today's date is 40,765. Today at noon will be 40,765.5