Hi, dawa!
First a little explanation of how Excel handles dates. It handles dates as integer numbers from 0 for 31/12/1899 (or 1 for 01/01/1900), and as it allows to perform mathematical operations with them (=A1+15 for adding 15 days to date in cell A1)
So first of all, you if you considered that point, you'd be retrieving a -424 value which is the difference in days between the two dates. Which it's right.
If you still have issues, consider uploading a sample file (including manual examples of desired output if applicable), it'd be very useful for those who read this and might be able to help you. Thank you. Give a look at the green sticky posts at this forums main page for uploading guidelines.
Regards!