A couple of things:

First, the formula should be "DATEDIF" vice "DATEIF".

Second, change the format of the cell with the formula to general instead of a date format. That way, it will show as "1" instead of "1/1/1900".

If you change the number in A6 to a "2" and the number in B6 to a "2" then that will represent Feb 2, and the formula should give you the result you were trying to find (33).

Cheers,