The spam entry brought me back here.
@pekoms... you never got back to me on what unit of measure you wanted the result to actually be.
Just in case you still have your ears on this thread, please see the following article on the subject.
I'll also tell you that while the difference of two datetimes appears to be stored as a date, it's actually stored as two integers behind the scenes but you don't have to worry about that. Just convert it to a FLOAT... the answer returned will appear as a decimal number that represents the number of whole 24 hour periods that have passed and the decimal portion represents the part of a 24 hour period as the "partial day" that has passed.
The article explains all of that and one way to display it. There are many other possibilities to display (and store) the duration as but we need to know what you want that to be. The cool part is, the converted number can be used with SUM and anything else that can handle plain ol' numbers.