在java里面如何计算日期

Question

Calendar a=new GregorianCalendar(2003,0,2);
           Calendar c=new GregorianCalendar(2003,0,4);
           System.out.println(a.getTime());
           a.add(Calendar.HOUR,2);
           System.out.println(a.getTime());
           System.out.println(a.get(Calendar.DAY_OF_YEAR));
           System.out.println(c.get(Calendar.DAY_OF_YEAR));
           System.out.println(a.get(Calendar.DAY_OF_YEAR)-c.get(Calendar.DAY_OF_YEAR));
        }