How does that work? Maybe if you run out of numbers in the nonce you get a new time?
Change "something". A bitcoin block header consists of:
Previous block hash
"Bits" (target in compact form)
Nonce (32-bit number)
There is no requirement to change the block header a particular way to attempt another hash but usually when you exhaust the nonce range you either
1) change extra nonce value in coinbase which results in a new merkle tree hash
2) change the timestamp
Pools usually do both to improve efficiency.
Each worker has a different merkle root and the pool allows the worker locally to increment time (n-time-rolling).