Hi G-OFUK,
Interesting initial question.
It is possible for a switch on my aircraft's clock to be to placed in the non normal "SET" position rather than the "GPS" position. From the write up (copied below), it would be possible to set a different time to GMT / UTC. I guess this clock time would be transmitted by ADC - but I'm not sure. Somebody else with ADC may be able to confirm.
From FCOM A320
"A fully independent clock is on the right side of the control panel.
It sends time to the centralized fault data interface unit, the flight data interface unit, and the flight management and guidance computer.
......
The clock performs four functions :
It displays "UTC" (GMT) time in hours, minutes and seconds on the center counter.
It displays elapsed time (ET) (from engine startup) in hours and minutes on the lower counter.
It drives the chronometer (CHR), which measures a time interval (from the pushing of the CHRONO button) in minutes and seconds.
It can replace the UTC with the date.
UTC selector
"GPS" : Time (or date, if selected) is displayed, and this data is synchronized on GPS information.
Note :
If the signal between the GPS and the clock is not detected, dashes are displayed. Only the "INT" and "SET" positions are then available.
If the signal is detected, but GPS data is invalid, the clock automatically runs on its internal time.
The clock will automatically resynchronize on the GPS information, as soon as the GPS data becomes available.
"INT" : Internal time (or date, if selected) is displayed.
Note :
The clock's internal time is always synchronized with the latest valid GPS information.
If there is no valid GPS information at power up, the internal time will be 00:00:00, until the clock is initialized, or until valid GPS information is present.
"SET" : Allows the internal time and date to be initialized. "