This paper describes an approach to incorporating the notion of expiration time into data management based on the relational model. Expiration times indicate when tuples cease to be current in a database. The paper presents a formal data model and a query algebra that handle expiration times transparently and declaratively. In particular, expiration times are exposed to users only on insertion and update, and when triggers fire due to the expiration of a tuple; for queries, they are handled behind the scenes and do not concern the user. Notably, tuples are removed automatically from (materialised) query results as they expire in the (base) relations. For application developers, the benefits of using expiration times are leaner application code, lower transaction volume, smaller databases, and higher consistency for replicated data with lower overhead. Expiration times turn out to be especially useful in open architectures and loosely-coupled systems, which abound on the World Wide Web as well as in mobile networks, be it as Web Services or as ad hoc and intermittent networks of mobile devices.