What stops miners/nodes lying about what time a block was mined?
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. &quot;Network-adjusted time&quot; is the median of the timestamps returned by all nodes connected to you.
Whenever a node connects to another node, it gets a UTC timestamp from it, and stores its offset from node-local UTC. The network-adjusted time is then the node-local UTC plus the median offset from all connected nodes. Network time is never adjusted more than 70 minutes from local system time, however.


http://en.bitcoin.it/wiki/Block_timestamp

So, if you want your block accepted by other nodes, you won't mess with anything close to deviating from the current time by two hours.
But the protocol doesn't care how many blocks are being mined, it only will use those durations of the past 2,016 blocks to adjust difficulty when that periodic event occurs.