The miner embeds their current (local) timestamp in the header, so there can be small variations. In fact the only requirement on the next timestamp is that it

The miner embeds their current (local) timestamp in the header, so there can be small variations. In fact the only requirement on the next timestamp is that it not be less than the median of the past 11 timestamps. Once embedded in the header, everyone will agree on past timestamps as they agree on the chain with the most accumulated difficulty.