A clock time adjustment system for a vehicle equipped with navigation system whereby the time on the system clock is adjusted based upon position of the vehicle and the time signal provided by the Global Positioning Satellite System (GPS). The navigation system is capable of determining vehicle position and the system memory is provided with data corresponding to regional time zones. The system determines the current position of the vehicle and then calculates the proper time for the vehicle's location using data corresponding to the regional time zone where the vehicle is currently located. The system also stores, into memory, road data having routes that are undesirable for travel at particular times which may then be used to adjust the trip route to the destination by comparing the road data and time to determine an optimal route that avoids roads that are undesirable for traveling during a specific period.