nmea0 huge timedelta while system clock is in sync

[Martin]

hw.sensors.nmea0.indicator0=On (Signal), OK
hw.sensors.nmea0.timedelta0=619313970.981246 secs (GPS autonomous), OK, Sun Mar 
22 12:47:08.981
               ^^^^^^
hw.sensors.nmea0.angle0=10.0 degrees (Latitude), OK
hw.sensors.nmea0.angle1=20.0 degrees (Longitude), OK
hw.sensors.nmea0.distance0=30.0 m (Altitude), OK
hw.sensors.nmea0.velocity0=0.000 m/s (Ground speed), OK

It works for about two years before like a charm, but now timedelta 
619313970.981246 secs.
Tried to change GPS receiver, no effect.

Martin