Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

var
   FirstWeekDay: Integer = 2;
   { Wochentag, mit dem die Woche beginnt 
   (siehe Delphi-Wochentage) 
   2 : Montag (nach DIN 1355) }
   FirstWeekDate: Integer = 4;
   { 1 : Beginnt am ersten Januar 
    4 : Erste-4 Tage-Woche (nach DIN 1355) 
    7 : Erste volle Woche }

   { liefert das Datum des ersten Tages der Woche }
   { get date of first day of week}
 function WeekToDate(AWeek, AYear: Integer): TDateTime;
 begin
   Result := EncodeDate(AYear, 1, FirstWeekDate);
   Result := Result + (AWeek - 1) * 7 - ((DayOfWeek(Result) + (7 - FirstWeekDay)) mod 7);
 end;

 { liefert die Wochennummer und das Jahr, zu dem die Woche gehort }
 { get weeknumber and year of the given week number}
 procedure DateToWeek(ADate: TDateTime; var AWeek, AYear: Word);
 var
   Month, Day: Word;
 begin
   ADate := ADate - ((DayOfWeek(ADate) - FirstWeekDay + 7) mod 7) + 7 - FirstWeekDate;
   DecodeDate(ADate, AYear, Month, Day);
   AWeek := (Trunc(ADate - EncodeDate(AYear, 1, 1)) div 7) + 1;
 end;


 {Week to date example}
 procedure TForm1.Button1Click(Sender: TObject);
 begin
   ShowMessage(FormatDateTime('dd.mm.yyyy', WeekToDate(51, 2000)));
 end;

 {Date to week example}
 procedure TForm1.Button2Click(Sender: TObject);
 var
   week, year: Word;
 begin
   DateToWeek(now, week, year);
   ShowMessage(IntToStr(week));
   ShowMessage(IntToStr(year));
 end;