20 minutes into the future is rather major. What's the use of the timestamp if it can't be relied upon?
I understand what you meant, but we're talking about P2P network where such accuracy can't be guaranteed and doesn't mean it can't be relied at all.
On a side note, according to
https://en.bitcoin.it/wiki/Block_timestamp, block time which accepted are
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. As a result block timestamps are not exactly accurate, and they do not need to be. Block times are accurate only to within an hour or two.