Press the 'TA' button (button to right of LCD display) for about 2 seconds till you hear a beep and clock starts flashing, then use 'H' ('1' button on program select) and 'M' ('2' button on prog select) to advance hour and minute to correct time. When you have finished press 'TA' button again till clock stops flashing.
I have to reset the clock in my Jazz GD quite often as it loses about a minute a fortnight!!