code: public static Date dateReservedDay000(Calendar calendar) {
          calendar.set(Calendar.HOUR_OF_DAY, 0);
          calendar.set(Calendar.MINUTE, 0);
          calendar.set(Calendar.SECOND, 0);
          calendar.set(Calendar.MILLISECOND, 0);
          return calendar.getTime();
      }
docs: 将一个日期的时,分,秒,毫秒置零
  
  @param calendar 一个日期
  @return 被转化后的日期
  @see #dateReservedDay000(Date)