First you have to convert the text (20040314) into a date that Excel can use.
If 20040314 is in cell A1 put this formula in another cell:
Now format that cell with a date format, or a custom format such as "mm/dd/yyyy" and the cell will display 03/14/2004.
If you want to delete the original, then copy the cell with the formula and Paste Special - 'Values and Number Formats', back into either the same cell or the cell that had the original text.
The new date information is now in a form recognized by Excel.
If you did the Paste Special back into cell A1 enter =A1 +1 in another cell which yields 03/15/2004, i.e., 1 day later.
You will be able to add and subtract dates to get the difference in days, and several other date functions, such as WEEKDAY(A1) which returns 1, and formatted as "dddd" it displays Sunday.