Thank you all for that information. There are ways of resolving the timestamp in a decentralized manner. when conducting the comparison of the size of the proof of work, each submitted proof of work will have an associated timestamp. Take the average and use that as the timestamp for the longest. There are also methods of synchronizing the network clock, using the clocktime of participants.
I'm not sure what you mean by "each submitted proof of work" and "when conducting the comparison of the size of the proof of work". As far as the Bitcoin network is concerned, there is no proof of work other than each block. In addition, the timestamp is part of the block header, meaning that it's part of what gets hashed in the PoW process. There are other ways of associating blocks with times, just like there are ways to associate transactions with times, but the block timestamp cannot be relied upon to be completely accurate.