Try this. It worked for me



shared datevar dtmTargetDate := date({PRE_CASE.START_DATE});
shared datevar dtmStartDate := date({PRE_CASE.START_DATE});
shared numbervar dtmDay := DatePart("d", dtmTargetDate);
shared numbervar dtmMonth := DatePart("m", dtmTargetDate);
shared numbervar dtmYear := DatePart("yyyy", dtmTargetDate);
shared numbervar intWeekday;
shared numbervar intAddon;
shared numbervar intWeek1;
shared numbervar intWeek2;
shared numbervar intWeek3;
shared numbervar intWeek4;
shared numbervar intWeek5;
shared numbervar intWeek6;
shared numbervar numWeek;

intWeekday := Weekday({PRE_CASE.START_DATE});
intAddon := 8 - intWeekday;

intWeek1 := intAddOn;
intWeek2 := intWeek1 + 7;
intWeek3 := intWeek2 + 7;
intWeek4 := intWeek3 + 7;
intWeek5 := intWeek4 + 7;
intWeek6 := intWeek5 + 7;

If dtmDay <= intWeek1 Then
    numWeek := 1
Else

If dtmDay <= intWeek2 Then
    numWeek := 2
Else

If dtmDay <= intWeek3 Then
    numWeek := 3
Else

If dtmDay <= intWeek4 Then
    numWeek := 4
Else

If dtmDay <= intWeek5 Then
    numWeek := 5
Else

If dtmDay <= intWeek6 Then
   numWeek := 6;

numweek;