web3.eth.getTransaction({txhash}) will contain a blockNumber.

Use [web3.eth.getBlock](https://github.com/ethereum/wiki/wiki/JavaScript-API#web3ethgetblock) to retrieve the block details and you will find the following field:

     timestamp: Number - the unix timestamp for when the block was collated.

This is a Unix timestamp.

Example using `geth` for the following transaction [0x5da2844afb6826d4baed6ad7e8b536c00cbc921ac147773ad056f29f2e7c1762](https://etherscan.io/tx/0x5da2844afb6826d4baed6ad7e8b536c00cbc921ac147773ad056f29f2e7c1762).

    > web3.eth.getTransaction("0x5da2844afb6826d4baed6ad7e8b536c00cbc921ac147773ad056f29f2e7c1762").blockNumber
    1920050
    > web3.eth.getBlock(1920050).timestamp
    1469021581

And using [www.unixtimestamp.com](http://www.unixtimestamp.com/index.php), this works out to be `07/20/2016 @ 1:33pm (UTC)` which matches the etherscan.io details.