if then formula help

I am trying to use the If-Then capability to compare the first cell with one specific value to a second cell that is already a formula using dates.  The second cell looks at two different dates and calculates the number of days between them.  Then, if the first cell has a specific word in it (from a pull down set of choices) i want the second cell to show that number of days that were calculated and if the first cell is any other word then to show 0 (zero) in the second cell.