[[duilioisola]]

Si es OBTENER, como dice roman...

De la ayuda de Delphi 6:

DaysInAMonth function
Returns the number of days in a specified month of a specified year.

Unit
DateUtils

Category
date/time routines

function DaysInAMonth(const AYear, AMonth: Word): Word;

Description
Call DaysInAMonth to obtain the number of days in the specified month of the specified year.
AYear is a year between 1 and 9999 (inclusive).
AMonth is a month between 1 and 12 (inclusive).

En DelphiBasics

Código Delphi [-]
begin
  // How many days in February 2000 ?
   ShowMessage('Days in February 2000 = '+
               IntToStr(DaysInAMonth(2000, 2)));
end;