A transaction doesn't have a timestamp. However, every block provides a timestamp (when it was collected), but if the time is critical for you, don't refer to this `timestamp` because a miner could modify it by about 900s. Instead, you could use `block.number`. 

1- current block timestamp is returned by `now`:

e.g.

    contract Test {
        function Time_call() returns (uint256){
            return now;
        }
    }
 
`Time_call()` will return something like `1478431966` (which you could convert into a readable form in [https://www.unixtimestamp.com/][1]).

2- block number is returned using `block.number`:
  

     function Time_call() returns (uint256){
            return block.number; 
        }

to get the call time, you could use the `block.number` and the [block time][2] (an average).

**Edit**: if you want when the transaction was sent, use JavaScript in your Dapp to get the current time `var seconds = new Date().getTime();` and send it in the data field within your transaction.

  [1]: https://www.unixtimestamp.com/
  [2]: https://etherscan.io/chart/blocktime