function purchaseMembership(uint256 costShareBenefit) external {
     ...
      if (membership[msg.sender].creation == 0) {
            ...
            membership[msg.sender].creation       = block.timestamp;
            membership[msg.sender].gracePeriod =  membership[msg.sender].creation +  MEMBERSHIP_DURATION +  60 days;
        } else {
          ....
          membership[msg.sender].creation += durationIncrease;
   }
