SQL> create or replace procedure check_date(v_date in date)
  2  is
  3    not_realistic exception;
  4    pragma exception_init(not_realistic,-20001);
  5  begin
  6    if v_date > sysdate Then
  7       raise not_realistic;
  8    end if;
  9  exception
 10    when not_realistic then
 11      raise_application_error(-20001,'A birth date cannot be in the future');
 12  end;
 13  /

Procedure created.

SQL> exec check_date(to_date('&ddmmyyyy','ddmmyyyy'))
Enter value for ddmmyyyy: 21012004
begin check_date(to_date('21012004','ddmmyyyy')); end;

*
ERROR at line 1:
ORA-20001: A birth date cannot be in the future
ORA-06512: at "MHE.CHECK_DATE", line 11
ORA-06512: at line 1

SQL> exec check_date(to_date('&ddmmyyyy','ddmmyyyy'))
Enter value for ddmmyyyy: 05122003

PL/SQL procedure successfully completed.

SQL> create table persons(name varchar2(255)
  2                      ,birth_date date
  3                      );

Table created.

SQL> CREATE TRIGGER BRIU_BIRTH_DATE
  2  BEFORE  INSERT  OR UPDATE  ON PERSONS
  3  FOR EACH ROW
  4  Declare
  5    invalid_date Exception;
  6    pragma exception_init(invalid_date,-20001);
  7  Begin
  8    If :new.birth_date > trunc(sysdate) Then
  9      Raise invalid_date;
 10    End If;
 11  Exception
 12    When invalid_date Then
 13      Raise_Application_Error(-20001,'A birth date cannot be in the future!');
 14  End;
 15  /

Trigger created.
<I>-- Insert a record with a birth_date in the past</I>
SQL> insert into persons values('Mike',SYSDATE-2);
<I>-- success!</I>
1 row created.

<I>-- Insert a record with a future birth_date</I>
SQL> insert into persons values('Valerie', SYSDATE+5);
<I>-- It fails!</I>
insert into persons values('Valerie', SYSDATE+5)
            *
ERROR at line 1:
ORA-20001: A birth date cannot be in the future!
ORA-06512: at "MHE.BRIU_BIRTH_DATE", line 10
ORA-04088: error during execution of trigger 'MHE.BRIU_BIRTH_DATE'