From Bitcoin.org
The peer-to-peer network allows block time to be up to two hours ahead of real time, so a locktime transaction can be added to the blockchain up to two hours before its time lock officially expires.
Why does the bitcoin blockchain allow blocks to have timestamps 2 hours ahead of real time (the real world clock time on a node)? What is the advantage of doing this?
Because not everyone's nodes' clocks are in sync with each other and because of network latency, there needs to be some buffer of time that a block's timestamp can be in so that nodes will still accept blocks.
Having a larger buffer allows for miners to change the timestamp too in addition to the nonce in order to have more possible blocks given the same list of transactions.