Author: Tomas Rutkauskas

How to count the number of Mondays between two given dates 

Answer:

Solve 1:

1   function NumMondays(dt1, dt2: TDateTime): integer;
2   var
3     Date1, Date2, DateSpan: integer;
4     Weekday1, DaysInStub: integer;
5     MondayInStub: Boolean;
6   begin
7     {Make sure date 1 is smaller than date 2}
8     Date1 := MinIntValue([Trunc(dt1), Trunc(dt2)]);
9     Date2 := MaxIntValue([Trunc(dt1), Trunc(dt2)]);
10    {First approximation: complete weeks}
11    DateSpan := Date2 - Date1 + 1;
12    result := DateSpan div 7;
13    {Now check if there's a Monday in the stub}
14    MondayInStub := false;
15    DaysInStub := DateSpan mod 7;
16    Weekday1 := DayOfWeek(Date1);
17    case Weekday1 of
18      {Sunday}
19      1: MondayInStub := DaysInStub > 0;
20      {Monday}
21      2: MondayInStub := true; {Starts and ends with Monday}
22      {Sunday}
23      3..7: MondayInStub := (Weekday1 + DaysInStub > 9 {2+7});
24    end;
25    if MondayInStub then
26      inc(result);
27  end;



Solve 2:

Something like this should do the trick. I included the variable setup and display 
of results from my little test so that it will be obvious what I did.

28  procedure TForm1.Button1Click(Sender: TObject);
29  var
30    cnt: integer;
31    StartDate, EndDate: TDate;
32  begin
33    cnt := 0;
34    StartDate := StrToDate('4/21/2003');
35    EndDate := StrToDate('5/30/2003');
36    {Actual Monday counting}
37    repeat
38      if DayOfWeek(StartDate) = 2 then {2 = Monday (Sun = 1 .. Sat = 7) }
39        inc(cnt);
40      StartDate := StartDate + 1;
41    until
42      StartDate = EndDate;
43    label1.Caption := IntToStr(cnt);
44  end;