uses DateUtils;
// ...
procedure TForm1.Button1Click(Sender: TObject);
var
dtBeg,
dtEnd : tDatetime;
intDays : integer;
begin
dtBeg := encodeDate( '1980', '01', '01' );
dtEnd := encodeDate( '2002', '09', '14' );
intDays := daysBetween( dtEnd, dtBeg );
messageBoxFmt( 'There are %d days between %s and %s.',
[ intDays, dateTimeToStr( dtBeg ),
dateTimetoStr( dtEnd ) ] );
end;