Transaction timestamps specify a [start, end] time window within which the transaction is asserted to have occurred. Timestamps are expressed as windows because in a distributed system there is no true time, only a large number of desynchronised clocks. This is not only implied by the laws of physics but also by the nature of shared transactions - especially if the signing of a transac- tion requires multiple human authorisations, the process of constructing a joint transaction could take hours or even days.