When you run ReadDept using the new class on the old object it throws
a NullPointerException because the SimpleDateFormat.format(Date)
method receives a null parameter when we pass departmentStartDate.

A newly constructed object will never have this problem because the
constructors provide a default value for departmentStartDate. However,
when we deserialize an old version of the class in which that field is
missing, it is set to null by the default deserialization mechanism.
