In a p2p environment, timestamps are totally useless at proving when I sent you bitcoins. However, when I sent you bitcoins is fundamentally important when it comes to ordering transactions, because I can send you bitcoins, but also send them to myself after you accept them. If I can forge timestamps then this attack is trivial to pull off.
You need an analogue to elapsed time to solve this problem. Hashes in bitcoin are probabilistic, there is an expected number of hashes required to solve the PoW (on average). Each hash takes an amount of time that cannot be made faster, so you suddenly have an unforgeable, easily provable analogue to elapsed time.
You need an analogue to elapsed time to solve this problem. Hashes in bitcoin are probabilistic, there is an expected number of hashes required to solve the PoW (on average). Each hash takes an amount of time that cannot be made faster, so you suddenly have an unforgeable, easily provable analogue to elapsed time.