Difference Between Two Day's

[#1]

How can I find out the difference between two days?? Including the holidays.

Example:

(08/01/2011     08/05/2011) = 4 days

[#2]

I am trying to subtract two date's and the result was displaying on chart was '-'. The data format are YYYY-MM-DD HH:MI:SS. Please let me know what I am missing hear.

Thanks.