excel spreadsheet that totals employee hours for the week

I am trying to create an excel spreadsheet that will total employee hours for the week. However, we run three shifts and some employees may have several shifts performed during the week. For example, worker one may work 7 am - 3 pm on Mon, then 3 pm- 11 pm on Wed, the 11 pm - 7 am on Sat. Total hours are 24. A simple sum of (B-A)*24+(D-C)*24+(F-E)*24 will not work because it reads the sat shift as a negative number and total hours become 8. I think i need an if statement that looks to see if b is greater than a or less than a. Have tried several formulas that have been posted on other sites by other "excel experts" but have had no luck. HELP!

Hi,
Format the date cells as "mm/dd/yyyy hh:mm" then
A1: 05/05/2013 18:00
A2: 05/06/2013 02:00 B2: =(A2-A1)*24 (answer = 8)

Normek