Around 24 January 2014 23:05 GMT not sure though.
To be more exact, the last block before the adjustment is Block #282239.
After it is found, the difficulty goes up.
Block #282239, Difficulty: 1789546951.05, Timestamp: 2014-01-24 13:25:11 (UTC)
https://blockchain.info/block-index/462830Block #282240, Difficulty: 2193847870.17, Timestamp: 2014-01-24 13:28:46 (UTC)
https://blockchain.info/block-index/462831