A difference in time from a standard time for each state, or where a state belongs to a plurality of time zones, a difference in time for each time zone and boundary information for specifying a state portion belonging to each time zone, are stored. In operation, a state in which a vehicle is present is detected, a check is made to determine whether the state in which the vehicle is present belongs to only one time zone by referring to the stored information, and if the state belongs to only one time zone, a time display is provided on the basis of the difference in time corresponding to that time zone. However, if the state in which the vehicle is present belongs to two time zones, a state portion where the vehicle is present is obtained on the basis of the boundary information and the vehicle's present position, and a time display is provided on the basis of the difference in time corresponding to the time zone associated with said state portion.