code: public static int getGrade(int year, int grade, Date onDate) {
  		Calendar calOnDate = Calendar.getInstance();
  		calOnDate.setTime(onDate);
  		int onDateYear = calOnDate.get(Calendar.YEAR);
  		if (calOnDate.get(Calendar.MONTH) < Calendar.SEPTEMBER)
  			onDateYear--;
  
  		grade += (onDateYear - year);
  		if (grade < 0)
  			grade = 0;
  		else if (grade > 13) // 0 = JK, 1 = SK, ... 13 = grade 12
  			grade = 13;
  
  		return grade;
  	}
docs: 学生在year年的9月份是grade年级，计算他在onDate是几年级