function releasableAmount(address _owner) public view returns (uint256){
        if (_owner == address(0)) {
            return 0;
        }
        TimeEnvoy storage owner = owners[_owner];
        if (owner.released) {
            return 0;
        } else if (block.timestamp >= owner.releaseTime) {
            return owner.balance;
        } else {
            return 0;
        }
    }