function DaysInMonth(const ADateTime: TDateTime): Byte;
const
  cDaysPerMonth: array[1..12] of Byte =  // array of days in months
    (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
  cMonthFeb = 2;            // month number of February
  cDaysInLeapYearFeb = 29;  // number of days in February in leap year
var
  Month: Word;  // month component of specified date
begin
  Month := DateMonth(ADateTime);
  Result := cDaysPerMonth[Month];
  if (Month = cMonthFeb) and IsLeapYear(ADateTime) then
    Result := cDaysInLeapYearFeb;
end;
//delphi/2170