[ContraVeneno]

Código Delphi [-]
var
  Present: TDateTime;
  Year, Month, Day, Hour, Min, Sec, MSec, Days: Word;
 begin
  Present:= Now;
  DecodeDate(Present, Year, Month, Day);
  dtpIni.DateTime:=EncodeDate(Year,Month,1);
  dtpFin.DateTime:= dtpIni.DateTime+DaysInAMonth(Year, Month);
  //o tambien
  dtpFin.DateTime:= DecodeDate(Present, Year, Month, DaysInAMonth(Year, Month);

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Que rapidez amigo Jhonny