function WeekOfYear(ADate : TDateTime) : word;

var

 day : word;

 month : word;

 year : word;

 FirstOfYear : TDateTime;

begin

 DecodeDate(ADate, year, month, day);

 FirstOfYear := EncodeDate(year, 1, 1);

 Result := Trunc(ADate - FirstOfYear) div 7 + 1;

end;



procedure TForm1.Button1Click(Sender: TObject);

begin

 ShowMessage(IntToStr(WeekOfYear(Date)));

end;

function WeekNum(const ADate: TDateTime): word;

var

 Year: word;

 Month: word;

 Day: word;

begin

 DecodeDate(ADate + 4 - DayOfWeek(ADate + 6), Year, Month, Day);

 result := 1 + trunc((ADate - EncodeDate(Year, 1, 5) +

  DayOfWeek(EncodeDate(Year, 1, 3))) / 7);

end;

function WeekOfYear(Dat: TDateTime): Word;

// Интерпретация номеров дней:

// ISO: 1 = Понедельник, 7 = Воскресенье

// Delphi SysUtils: 1 = Воскресенье, 7 = Суббота

var

 Day,

 Month,

 Year: Word;

 FirstDate: TDateTime;

 DateDiff : Integer;

begin

 day := SysUtils.DayOfWeek(Dat)-1;

 Dat := Dat + 3 - ((6 + day) mod 7);

 DecodeDate(Dat, Year, Month, Day);

 FirstDate := EncodeDate(Year, 1, 1);

 DateDiff := Trunc(Dat - FirstDate);

 Result := 1 + (DateDiff div 7);

end;

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;