code: public static int getMonthLength(@NotNull final Date date) {
  		int year = get(date, Calendar.YEAR);
  		int month = get(date, Calendar.MONTH);
  		return getMonthLength(year, month);
  	}
docs: 获取某个月有多少天, 考虑闰年等因数, 移植Jodd Core的TimeUtil