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) div7 + 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) mod7);

DecodeDate(Dat, Year, Month, Day);

FirstDate := EncodeDate(Year, 1, 1);

DateDiff := Trunc(Dat - FirstDate);

Result := 1 + (DateDiff div7);

end;