code: public static Date dateReservedMonth000(Calendar calendar) {
          calendar.set(Calendar.DAY_OF_MONTH, 1);
  
          return dateReservedDay000(calendar);
      }
docs: 将一个日期的日,时,分,秒,毫秒调整为最小值
  
  @param calendar 一个日期
  @return 被转化后的日期
  @see #dateReservedMonth000(Date)