code: public static TimeUnit getParentUnit(TimeUnit unit)
      {
          TimeUnit result = null;
  
          switch (unit)
          {
          case HOURS:
              result = TimeUnit.DAYS;
              break;
          case MINUTES:
              result = TimeUnit.HOURS;
              break;
          case SECONDS:
              result = TimeUnit.MINUTES;
              break;
          case MILLISECONDS:
              result = TimeUnit.SECONDS;
              break;
          case MICROSECONDS:
              result = TimeUnit.MILLISECONDS;
              break;
          case NANOSECONDS:
              result = TimeUnit.MICROSECONDS;
              break;
          default:
              break;
          }
  
          return result;
      }
docs: 指定したTimeUnitより１つ大きい単位のTimeUnitを取得する。
  
  @param unit 時間単位
  @return １つ大きい単位のTimeUnit