Transactions do not have an individual timestamp. The closest you can do is get the timestamp from the block that it was included in (but note Pieter's warning about accuracy).

API Example

Using blockchain.info, for example you can do:

https://blockchain.info/rawtx/$tx_hash

For $tx_hash = b6f6991d03df0e2e04dafffcd6bc418aac66049e2cd74b80f14ac86db1e3f0da this would return:

{
    "hash":"b6f6991d03df0e2e04dafffcd6bc418aac66049e2cd74b80f14ac86db1e3f0da",
    "ver":1,
    "vin_sz":1,
    "vout_sz":2,
    "lock_time":"Unavailable",
    "size":258,
    "relayed_by":"64.179.201.80",
    "block_height: 12200,
    "tx_index":"12563028",
    "inputs":[


            {
                "prev_out":{
                    "hash":"a3e2bcc9a5f776112497a32b05f4b9e5b2405ed9",
                    "value":"100000000",
                    "tx_index":"12554260",
                    "n":"2"
                },
                "script":"76a914641ad5051edd97029a003fe9efb29359fcee409d88ac"
            }

        ],
    "out":[

                {
                    "value":"98000000",
                    "hash":"29d6a3540acfa0a950bef2bfdc75cd51c24390fd",
                    "script":"76a914641ad5051edd97029a003fe9efb29359fcee409d88ac"
                },

                {
                    "value":"2000000",
                    "hash":"17b5038a413f5c5ee288caa64cfab35a0c01914e",
                    "script":"76a914641ad5051edd97029a003fe9efb29359fcee409d88ac"
                }

        ]
}

From there you can query the block height of the inputs the same way as above using the inputs[i].prev_out.hash field. Then, to get the timestamp for a block, just query the raw block by height. E.g. for the output timestamps, you would use block_height: 12200 for the example above:

https://blockchain.info/block-height/12200?format=json

The timestamp would be returned in blocks[0].time

Note: Transactions technically can have an individual timestamp. When they do, it is the locktime property, see Bitcoin Wiki - Transaction. However, the locktime is most likely not going to be the time that it was created, rather, the time it is available for spending:

if non-zero and sequence numbers are < 0xFFFFFFFF: block height or timestamp when transaction is final