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 you could use instead `block.number`. 

1- current block timestamp is returned by now :

e.g

    contract Test {
        function Time_call() returns (uint256){
            return now;
        }
    }
 
Time_call will returns something like : 1478431966 (which you could convert in a readble form in [http://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 avarage)

**Edit**: if you want when the transaction was sent use javascript in your Dapp to get the current time `var seconds = new Date().getTime() / 1000;` and send it in the data field withing your transaction.

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