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 user schedule data from at least one user device, and to obtain destination data representative of a destination and a desired arrival time from the user schedule data; determine an expected travel time to the destination in dependence upon historic travel data, wherein said historic travel data comprises measured average speed data at different times; determine a departure time in dependence upon the expected travel time to the destination such that the user arrives at the destination by the desired arrival time; recalculate the expected travel time in dependence upon current travel data when the determined departure time is within a predetermined threshold of the current time, wherein the current travel data is indicative of current travel conditions; and update the departure time in dependence upon the recalculated expected travel time.