Although temporal databases have received considerable attention as a topic for research, little work in the area has paid attention to the concurrency control mechanisms that might be employed in temporal databases. This paper describes how the notion of the current time - also called now - in valid-time databases can cause standard serialisation theory to give what are at least unintuitive results, if not actually incorrect results. The paper then describes two modifications to standard serialisation theory which correct the behaviour to give what we term perceivably instantaneous transactions: transactions where serialising T1 and T2 as [T1,T2] always implies that the current time seen by T1 is less than or equal to the current time seen by T2.