code: public static Date firstDayOfMonth(Date date) {
          Calendar cal = Calendar.getInstance();
          cal.setTime(date);
          int value = cal.getActualMinimum(Calendar.DAY_OF_MONTH);
          cal.set(Calendar.DAY_OF_MONTH, value);
          return cal.getTime();
      }
docs: 返回传入时间月份的第一天