Timestamps are checked and enforced by notary services. As the participants in a notary service will themselves not have precisely aligned clocks, whether a transaction is considered valid or not at the moment it is submitted to a notary may be unpredictable if submission occurs right on a boundary of the given window. However, from the perspective of all other observers the notary’s signature is decisive: if the signature is present, the transaction is assumed to have occurred within that time.