СoW_token2.sol

// WRITE
function start() public onlyOwner {
  eventStartTime = now;
}

function transfer(address _to, uint256 _value) public returns (bool) {
  updateCopyOnWriteMaps(msg.sender, _to);
  return super.transfer(_to, _value);
}

function updateCopyOnWriteMaps(address _from, address _to) internal {
  if(!isBalanceWasChangedAfterEventStarted(_for)){
    balancesAtTheStartOfVoting[_for].balance = balances[_for];
    balancesAtTheStartOfVoting[_for].lastUpdateTime = now;
  }

  if(!isBalanceWasChangedAfterEventStarted(_for)){
    balancesAtTheStartOfVoting[_to].balance = balances[_to];
    balancesAtTheStartOfVoting[_to].lastUpdateTime = now;
  }
}


// READ
function getBalanceAtEventStart(address _for) public view returns(uint256){
  if (isBalanceWasChangedAfterEventStarted(_for)) {
    return balancesAtTheStartOfVoting[_for].balance;
  }
 
  return balances[_for];
}

function isBalanceWasChangedAfterEventStarted(address _for) internal view returns(bool) {
  return (balancesAtTheStartOfVoting[_for].lastUpdateTime >= eventStartTime);
}