How to compare the difference between two dates in Delphi?

Question

I need to compare 2 dates to know if the due date is Greater that the current date, if it is Minor it returns an error, follows the code I did and did not work.

 var
 Data_Atual: String;
 Data_vencimento: String;
 begin
 Data_Atual := DateToSTr(Date);
 Data_Vencimento := '20/02/2014';
 if (Data_Vencimento < Data_Atual);
 showmessage('A Data de Vencimento é menor que a data atual');
 end;

What I did wrong ?

Answer 1

Only one precaution to take: if you use Strtodate('20/02/2014') and the user’s machine is default dates in English, it will give error.

Whenever comparing dates in string format, you should invert the formatting. It should stay year / month / day. OK?

Code example:

var
   Data_Atual: string;
   Data_vencimento: string;
 begin
   Data_Atual := FormatDateTime('yyyy/mm/dd', Date);
   Data_Vencimento := '2014/02/20';
   if (Data_Vencimento < Data_Atual) then
     ShowMessage('A Data de Vencimento é menor que a data atual');
 end;

Answer 2

Use the function decodeDate().

In it, you pass 4 parameters:

• The first is the date;
• In the second you pass a variable to where the function will extract the date YEAR;
• In the third you pass a variable to where the function will extract the date MES;
• In the room you pass a variable to where the function will extract the date DAY.

In your case, you call this procedure twice and subtract the value of the two days. So or so:

    var
    DataInicial, DataFinal : TDateTime;
    AnoInicial, AnoFinal, MesInicial, MesFinal, DiaInicial, DiaFinal : Word;
    DiferencaEntreDias : Integer;

    Begin

      decodeDate(DateFinal, AnoFinal, MesFinal, DiaFinal);
      decodeDate(DataInicial, AnoInicial, MesInicial, DiaInicial);

      DiferencaEntreDias := DiaFinal - DiaInicial;

    end;

Answer 3

As quoted in Heber’s response you can compare dates through the operators > and < besides = to check if they match.

Var
 DataAtual, DataVencimento: String;
Begin
 DataAtual := FormatDateTime('dd/mm/yyyy', Date);
 DataVencimento := '20/02/2014';

 If (DataVencimento = DataAtual) then
   ShowMessage('A Data atual coincide com a Data de Vencimento!')

 Else If (DataVencimento > DataAtual) then
   ShowMessage('A Data Atual é superior a Data de Vencimento!')

 Else If DataVencimento < DataAtual then
   ShowMessage('A Data Atual é inferior a Data de Vencimento!')
End;

If you prefer, there is also a library that provides some functions that allow you to work with dates/hours in a more precise way, the DateUtils.

Examples

To compare the difference between two dates, use the function CompareDate:

Var
 DataAtual, DataVencimento: TDate;
 Value: Integer;
Begin
 DataAtual := StrToDate(FormatDateTime('dd/mm/yyyy', Now));
 DataVencimento := StrToDate('20/02/2014');

 Value := CompareDate(DataAtual, DataVencimento);
 If Value = 0 then
    MessageDlg('A Data Atual coincide com a Data de Vencimento!', mtInformation, [mbOK], 0)
 Else If Value = 1 then
    MessageDlg('A Data Atual é superior a Data de Vencimento!', mtInformation, [mbOK], 0)
 Else
    MessageDlg('A Data Atual é inferior a Data de Vencimento!', mtInformation, [mbOK], 0);
End;

To know the difference of days between two dates use the function DaysBetween:

Var
DataAtual, DataVencimento: TDate;
Diff: String;
Begin
 DataAtual := StrToDate(FormatDateTime('dd/mm/yyyy', Now));
 DataVencimento := StrToDate('20/02/2014');
 Diff := FloatToStr(DaysBetween(DataAtual, DataVencimento));

 MessageDlg(Format('A diferença entre %s e %s é de %s dias', 
       [DateToStr(DataAtual), DateToStr(DataVencimento), Diff]), 
       mtInformation, [mbOK], 0);
End;

To know the difference of weeks between two dates use the function WeeksBetween:

Var
DataAtual, DataVencimento: TDate;
Diff: String;
Begin
 DataAtual := StrToDate(FormatDateTime('dd/mm/yyyy', Now));
 DataVencimento := StrToDate('20/02/2014');
 Diff := FloatToStr(WeeksBetween(DataAtual, DataVencimento));

 MessageDlg(Format('A diferença entre %s e %s é de %s semanas', 
       [DateToStr(DataAtual), DateToStr(DataVencimento), Diff]), 
       mtInformation, [mbOK], 0);
End;

To know the difference of months between dates use the function MonthsBetween, for years use YearsBetween.

Related: Calculation of dates in Firebird