I personally would use a MAX7219 chip that can drive 8x 7 segments. See here for info Arduino Playground - LEDMatrix and couple this with an accurate RTC module based on something like the DS3231 (Not DS1307) http://macetech.com/store/index.php?main_page=product_info&cPath=5&products_id=8&zenid=45a03002f11feb37517a46a75dc36fa1 Battery backup of time from something like this is a must as chances of device running for years without power glitch is slim.