Missing segments are often the result of over powered or under powered batteries.
1. Please remove all power from the display (and sensor if you have one) for 2 hours.
2. With the batteries removed, press any button 20 times to clear all electricity from the unit. Please do this even if the screen is blank.
3. After 2 hours install fresh batteries (into sensor first, if you have a sensor) into the display.
4. Wait at least 15 minutes before pressing buttons.
5. Set the time and the date, then see if you still have missing segments.