Why the timestamp component of the block header? What is the reason for the timestamp field in the block header? Surely the nonce should be the only property that changes? Should the nonce reset to 0 everytime the timestamp changes?
The timestamp exists so there will be a permanent record of when the block was found. The timestamp needs to pass some sanity tests for the block to be considered valid.

One of the key uses of the timestamp is in calculating difficulty retargets.