Why make defensive copies in getters inside immutable classes?

Question

This question is about good programming practices and avoiding potential holes.
I read Joshua Bloch\'s Effective Java and here is what I wonder:
Why should I conside

Answer 1

I slightly modified Tomasz Nurkiewicz answer, to demonstrate why making dateOfBirth final does not defend it from being changed by a client class:

    public class Immutable {

        private final String name;
        private final Date dateOfBirth;

        public Immutable(String name, Date dateOfBirth) {
            this.name = name;
            this.dateOfBirth = dateOfBirth;
        }

        public String getName() { return name;  }

        public Date getDateOfBirth() { return dateOfBirth;  }

        public static void main(String[] args) {

            //create an object
            Immutable imm = new Immutable("John", new Date());

            System.out.println(imm.getName() + " " + imm.getDateOfBirth());

            //try and modify object's intenal
            String name = imm.getName(); //safe because Integer is immutable
            name = "George";             //has no effect on imm

            Date dateOfBirth = imm.getDateOfBirth();
            dateOfBirth.setTime(0);  //we just modified `imm` object

            System.out.println(imm.getName() + " " + imm.getDateOfBirth());
        }
    }

    **Output:** 
    John Wed Jan 13 11:23:49 IST 2016
    John Thu Jan 01 02:00:00 IST 1970