The idea of Bitcoin is that anyone who has knowledge of the protocol can log on and determine the correct state of the ledger trustlessly. I ask my peers for the current blockchain and choose the one that is both valid and has the most work.

However, this ideal is not entirely achieved. A block is only valid if its timestamp is close enough to the actual time. This means that malicious miners can confirm blocks with erroneous timestamps, and there would be no way for someone logging on for the first time to detect that this has happened. This is in stark contrast to the case of malicious miners confirming blocks with erroneous transactions, for these erroneous transactions can be detected using only the information internal to the blockchain.

Malicious miners can validate a block with an erroneous timestamp. If they are in the majority, their chain will have the most work. Anyone logging onto the network for the first time will not be able to detect that the time stamps are erroneous. There may be nodes running with no downtime who claim that the time stamps are indeed erroneous, but there is no way to confirm who is telling the truth.

The implication is precisely that the timestamps may have been forged and there is no way to detect this trustlessly. People talk as if blockchains can function as trustless clocks, but this is simply not the case.