Through a synchronization protocol but it's obviously impossible to have perfectly synchronized clocks so the safest thing to do would be to use several timesta

Through a synchronization protocol but it's obviously impossible to have perfectly synchronized clocks so the safest thing to do would be to use several timestamps from several clocks. Blockchains in a way are such clocks where the ordering of the blocks on the chain provide an implicit counter. But blockchains have too much overhead so by assuming some trust it would be possible to get rid of the proof of work mechanism and just use a signed timestamp from a trusted source.