1. A navigation system comprising: a processing resource and an output device for providing a departure time to a user, wherein the processing resource is configured to: receive destination data representative of a destination; determine, for each of a plurality of possible routes to the destination, a reliability measure representative of an expected reliability of an expected travel time for the route; select a route in dependence upon the reliability measures for the routes; determine an expected travel time to the destination based on the selected route; and determine the departure time in dependence upon the expected travel time to the destination, the processing resource being further configured to: receive an indication whether an appointment associated with the destination is of high importance; and, when the appointment is of high importance, select a route only from amongst routes having a reliability measure above a predetermined threshold.