web3.eth.getTransaction({txhash})
will contain a blockNumber.
Use web3.eth.getBlock 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.
> let tx = "0x5da2844afb6826d4baed6ad7e8b536c00cbc921ac147773ad056f29f2e7c1762"
> web3.eth.getTransaction(tx).blockNumber
1920050
> web3.eth.getBlock(1920050).timestamp
1469021581
And using www.unixtimestamp.com, this works out to be 07/20/2016 @ 1:33pm (UTC)
which matches the etherscan.io details.