Don't Supply a Mutator for every Accessor

• Day has getYear, getMonth, getDate accessors
• Day does not have setYear, setMonth,setDate mutators
• These mutators would not work well
  • Example:
    

    Day deadline = new Day(2001, 1, 31);
    deadline.setMonth(2); // ERROR

  • Example:

    Day deadline = new Day(2001, 2, 28);
    deadline.setDate(31); // ERROR

• GregorianCalendar implements confusing rollover. 
  • Silently gets the wrong result instead of error.
• Immutability is useful