EDIT: To be honest, knowledge of the hash of a block can be used as a proof that something has happened AFTER a particular event (similarly to a picture of a hostage with the newspaper).
But you can't prove that something has happened BEFORE a particular event using Bitcoin hashes.
Also, this "feature" is unrelated to Bitcoin being a cryptocurrency. You can have cryptocurrencies without "global time" (e.g. NANO).
But you can't prove that something has happened BEFORE a particular event using Bitcoin hashes.
Also, this "feature" is unrelated to Bitcoin being a cryptocurrency. You can have cryptocurrencies without "global time" (e.g. NANO).