Determining the Last Day of a Month

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		Determining the Last Day of a Month
		by Lance Leonard	Posted: 1 April 2002
		Submitted by Kathy M.	Applies to: All versions
			Audience: Intermediate

Question: I'm trying to determine the last day of a month, but am finding it difficult to implement all the rules associated with the different months, leap years, and so on. Isn't there an easy way to do this?

Actually, there is. Determine the first day of the next month and then subtract one day, as shown in the following code sample:

function getLastDay( dtADate : TDateTime ) : TDateTime;
// ---------------------------------------------------------
// When passed a value TDateTime, returns the last day of
// the month it refers to.  Note: Default exception handling
// of bad dates has not been overridden.  GIGO.
// ---------------------------------------------------------
var
   wYY, wMM, wDD : Word;      // Year, Month, & Day of input
begin

   decodeDate( dtADate, wYY, wMM, wDD );
   inc( wMM );
   if ( wMM = 12 ) then
   begin
      wMM := 1;
      inc( wYY );
   end;
   result := encodeDate( wYY, wMM, 01 ) - 1;

end;

As you can probably tell, the above sample provides a convenient function that does this for you. Use something like this to call it:

procedure TForm1.Button3Click(Sender: TObject);
var
   strLastDay : String;
begin

   strLastDay := DateTimeToStr( getLastDay( sysUtils.Date ) );
   application.MessageBox( pChar( strLastDay +
      ' is the last day of the current month.' ) ,
      'FYI', mb_OK );

end;

Tip: Because Date() and Time() are commonly used terms, it's a good idea to specifically qualify references to the functions in the SYSUTILS unit, as shown in the last code sample. This may help prevent confusion in the long run.

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Article last updated on 31 May 2003

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