The formula to calculate the target of a block requires the current timestamp, and also every block has the current timestamp attached to its header. Nothing can stop a miner from using some other timestamp instead of the current timestamp while mining a new block, but they don't usually because timestamp validation would fail and other nodes won't accept the block, and it would be a waste of resources of the miner. When a miner broadcasts a newly mined block, its timestamp is validated by checking whether the timestamp is greater than the timestamp of the previous block. If a miner uses a timestamp greater than the current timestamp, the difficulty will be low as difficulty is inversely proportional to the current timestamp; therefore, the miner whose block timestamp is the current timestamp would be accepted by the network as it would have a higher difficulty. If a miner uses a timestamp greater than the previous block timestamp and less than the current timestamp, the difficulty would be higher, and therefore, it would take more time to mine the block; by the time the block is mined, the network would have produced more blocks, therefore, this block will get rejected as the blockchain of the malicious miner will have a lower difficulty than the blockchain the network has. Due to these reasons, miners always use accurate timestamps, otherwise they gain nothing.