Ralph: Although I'm not familiar with your Garmin, make sure all other modes of your chart plotter are working, then check to see if your plotter "clock" is set to the right time. (Daylight saving time) If it still doesn't work, call Ed at Phantom Marine and I'll bet he can tell you what's wrong over the phone.
You might try updating the firmware on the unit, Garmin has software that makes it easy. If you have a laptop without a serial port you can pick up a USB to serial converter for less than $20