Date Calculation On Form In Days

[Jazzy007]

Hi
I have 3 date fields. I am trying to work out number of days when

=[FIELD1 - FIELD2 OR FIELD3]

should do the calculation in field 4 NUMBER OF DAYS. but it comes up with date only. FIELD1 will always be filled, if field 2 is field then the number of days must be based on that. but if field2 is blank then it should be field1 - field 3. I may need a help on the formula does it need any if, else statement or not. If so can someone please help

 

[RuralGuy]

If IsDate([Field2]) Then
   DateDiff("d",[Field1],[Field2])
Else
   DateDiff("d",[Field1],[Field3])
End If