Code:

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;

Code:

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;

Code:

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;

Code:

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;