|Did you copy your dates from somewhere or enter them by hand?|
If you copied them, make sure they were not read as TEXT:
First format the cell with the date as Number,
if it is a real date field you should get the serial number of the date,
for instance the date today is 7/19/2010 so formatting it as a number should give you 40378 which is the number of days from January 1, 1900.
If you don't get a number, then it's been formatted as TEXT.
To convert the TEXT date use the =DATEVALUE() function which returns the serial number of the date that is represented by the text date.