Simple License Expiration Example

Expiration.sol

pragma solidity ^ 0.4.4;

// *****************************************************************
// DEMO!! UNVETTED CODE! DO NOT DEPLOY!!!
// Please seek expert guidance and perform a solidity security audit
// before using it in financially important code.
// *****************************************************************

contract ArtLicenseExpiration {

  address public artwork; // the artwork -- artist has the public/private key
  address public buyer;   // the buyer of the piece -- who bought the piece
  uint256 expirationDate = 1544590800; // 12/12/2018 @ 12:00pm (UTC)
 
  constructor(address _buyer) public {
  
    // immutible -- you could make this modifiable if
    // you were to sell the art to another buyer
    buyer = _buyer; 
                    
    artwork = msg.sender;
  }


  function isExpired() public returns(bool) {
    require(msg.sender == artwork, "Only the art can call this function.");
    
    // use of "now": "now" does not mean current time.
    // Now is an alias for block.timestamp. Block.timestamp can be influenced
    // by miners to a certain degree, be careful.
    return (now >= expirationDate);
  }
 
  
  function setExpirationDate() public(uint256 _expirationDate) {
      require(msg.sender == artwork, "Only the art can call this function.");
      expirationDate = _expirationDate;
  }
 
 
  function() payable {
  
    // maybe anyone can renew the license? The buyer would still own it
    require(msg.sender == buyer, "Only the buyer can call this function."); 

    // here is where you calculate how much you want to charge for time.
    // in this example the artwork is worth 1 Wei = 1 second of time
    // this is a VERY expensive piece :)
    expirationDate += msg.value;
  }
}