Using Gantt chart / date subtractions

Hi I'm having a little issue.


So I have 4 columns: actual duration (day), actual start (date), budget start (date) and end date (date).

I need to find a formula to find the actual duration based on the three dates.
For example:

Actual Duration Actual Start Budget Start End Date
? 1/1/10 5/1/10 12/1/10

The formula MUST contain the 3 dates.


Thanks.