A navigation system for use in a vehicle receives a GPS signal for having a GPS time data that rolls over at an interval of 19.7 years with a start time of 0 a.m. Jan. 6th, 1980 from a GPS satellite, and acquires map generation date information that specifies map data generation time from map database in the navigation system. The GPS time data and the map data generation time information are utilized for calculating an accurate current date and time.