There is an example of this between the CR help files. You will need CR8.0 or above to use the DateDiff function
//Crystal syntax
Local DateTimeVar d1 := {Orders.Order Date};
Local DateTimeVar d2 := {Orders.Ship Date};
DateDiff ("d", d1, d2) -
DateDiff ("ww", d1, d2, crSaturday) -
DateDiff ("ww", d1, d2, crSunday)