set serveroutput on;
<<outer>>
  DECLARE
    birthdate DATE;
  BEGIN
  
      birthdate:=TO_DATE('2012-05-18','YYYY-MM-DD');
      DECLARE
         birthdate DATE;
      BEGIN
         birthdate:=TO_DATE('2012-01-15','YYYY-MM-DD');
         dbms_output.put_line('inner date is:' || birthdate);
         dbms_output.put_line('outer date is:' || outer.birthdate);
      END;
      
      dbms_output.put_line('outer date is:' || birthdate);
  END;

anonymous block completed
inner date is:15-JAN-12
outer date is:18-MAY-12
outer date is:18-MAY-12