Yes You can:
1. Add a method in Your form named CalculateDays
2. Add the followin code in that method:

  LPARAMETERS startdate, enddate
  LOCAL cycle, tocycle, testdate, ret_val
  ret_val = 0
  tocycle = enddate-startdate
  FOR cycle = 0 TO tocycle
      testdate = startdate + cycle
      IF .NOT. INLIST(DOW(ScanDay,1), 6, 7)  && friday, saturday
         ret_val = ret_val + 1
      ENDIF
  ENDFOR

RETURN ret_val

3. Change the lastline of Yourcode:

thisform.text3.value=thisform.CalculateDays(thisform.text1.value,thisform.text2.value)

BTW neither Function not Method or procedure can Have parameters with names thisform.xxxx
So You can't change FUNCTION xxx(startdate, enddate) to FUNCTION xxx(thisform.text1.Value, thisform.Text2.Value), or
LPARAMETERS thisform.text1.Value, thisform.Text2.Value





>hi,
>thank you very much,
>
>but i don't know how i can use it at myform.
>
>can i change code as under
>

>*thisform.text1.value=startdate
>*thisform.text2.value=enddate
>thisform.text3.value=CalculateDays(thisform.text1.value,thisform.text2.value)
>
>
>FUNCTION CalculateDays(thisform.text1.value, thisform.text2.value)
>  LOCAL cycle, tocycle, testdate, ret_val
>  ret_val = 0
>  tocycle = enddate-startdate
>  FOR cycle = 0 TO tocycle
>      testdate = thisform.text1.value+cycle
>      IF .NOT. INLIST(DOW(ScanDay,1), 6, 7)  && friday, saturday
>         ret_val = ret_val + 1
>      ENDIF
>  ENDFOR
>
>RETURN ret_val
>
>

>
>thanks,