function setTimes(
    uint256 _openingTime,
    uint256 _closingTime
  )
  public
  onlyOwner
  {
    require(
      _closingTime == 0 || _closingTime >= _openingTime,
      "wrong value for _closingTime"
    );

    openingTime = _openingTime;
    closingTime = _closingTime;
  }