1. A navigation device, comprising: a processor; and a memory store containing map data, the map data comprising one or more temporally-variant features and predetermined temporal information for each of the one or more temporally-variant features, wherein each temporally-variant feature is a road, a point of interest, or a map location and each temporally-variant feature has at least one property that changes with time in a manner that is known in advance and is defined in corresponding predetermined temporal information, and wherein said predetermined temporal information includes: (i) a start time of a variation in the map data associated with the corresponding temporally-variant feature and an end time of the variation in the map data; or (ii) a start time of the variation in the map data and duration of the variation in the map data, the processor being arranged to: determine a route from a start location to a destination location in a route planning process, wherein said determining comprises: determining, based on the predetermined temporal information for at least one of the temporally-variant features in the map data, whether temporally-variant weightings associated with the at least one temporally-variant feature are to be applied to the map data; determining a temporally-variant weighting for each road in a vicinity of the at least one temporally-variant feature based upon a distance of each road from the at least one temporally-variant feature; and upon determining that the temporally-variant weightings associated with the at least one temporally-variant feature are to be applied to the map data, when determining the route, applying the temporally-variant weightings for the roads in the vicinity of the at least one temporally-variant feature; and provide the route determined in the route-planning process.