Keep playing with the button.
Turn on the key.
Press the button.
Press it again.
Press it again.
Until the clock display illuminates.
OR
Press and hold the button until the display cycles through to get the hours flashing.
Release the button.
Press and hold to change the hours.
Press and hold again to get to the minutes.
OR
After changing the hours, turn off the key.
Then turn the key back on and cycle through to the minutes.
Change them and turn off the key.
Is there a button under the tachometer in the front?
Is there a position the key has to be turned to, to adjust the clock?
Sorry I can't be of any help. The bike was so rare, I haven't worked on many.