Determining the week number when given a TDateTime

I have seen this one asked quite a few times: How do you get the week number of a given date? Pass a TDateTime variable to CalendarWeek() below and it will calculate and return the week number. Thanks to Ralph Friedman for this code.

function CalendarWeek(ADate: TDateTime): integer;
{ Code adapted from Ralph Friedman (TeamB) <[email protected]> }
{ Calculates calendar week assuming:
- Monday is the 1st day of the week.
- The 1st calendar week is the 1st week
of the year that contains a Thursday.
If result is 53, then previous year is assumed. }
var
  firstOfYear: TDateTime;
  day,
  dayOne,
  month,
  monthOne,
  year: word;
begin
  DecodeDate(ADate, year, month, day);
  dayOne := 0;
  case DayOfWeek(EncodeDate(year, 1, 1)) of
    1: dayOne := 2; // Sunday
    2: dayOne := 1; // Monday
    3: dayOne := 31; // Tuesday
    4: dayOne := 30; // Wednesday
    5: dayOne := 29; // Thursday
    6: dayOne := 4; // Friday
    7: dayOne := 3; // Saturday
  end;

  if dayOne > 4 then
    begin
      Dec(year);
      monthOne := 12
    end
  else
    monthOne := 1;

  firstOfYear := EncodeDate(year, monthOne, dayOne);
  if ADate < firstOfYear then
    Result := 53
  else
    Result := (Trunc(ADate - firstOfYear) div 7) + 1;
end;


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