The timestamp can be up to two hours different. Block thus won't always be in chronological order for the timestamp.
Why? (How can this be?)
From the Bitcoin wiki:
Each block contains a Unix time timestamp. In addition to serving as a source of variation for the block hash, there are also validity checks, that make it more difficult for an adversary to manipulate the block chain.
A timestamp is accepted as valid if it is greater than the median timestamp of previous 11 blocks, and less than the network-adjusted time + 2 hours. "Network-adjusted time" is the median of the timestamps returned by all nodes connected to you.
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http://en.bitcoin.it/wiki/Block_timestamp
