Function:   DTDIFF( <expD1>, <expD2>, <expN> ) --> integer

Synopsis:   calculates the difference between two given dates depending
            upon action chosen.

            <expD1> start date

            <expD2> end date

            <expN>  action as defined in VISA.CH:
                            DT_DAY   = in days
                            DT_WEEK  = in weeks
                            DT_MONTH = in months
                            DT_YEAR  = in years

Examples:   lDate1  := CTOD( "10/15/90" )
            lDate2  := CTOD( "10/26/90" )
            diffdays  := DTDIFF(adate,adate2, DT_DAY)
            diffweeks := DTDIFF(adate,adate2, DT_WEEK)
            diffmonth := DTDIFF(adate,adate2, DT_MONTH)
            diffyears := DTDIFF(adate,adate2, DT_YEAR)

Returns:    integer representing the difference of both dates
            depending upon action chosen.