If it's a UK car, then hold down both the small buttons together for a
while. That should trigger the clock to be set using RDS data from the
radio.

If not, then for SID 1:-

select "temp"

Hold down CLEAR and arrow-down buttons for over a second - "Year"
should start to flash.

Press CLEAR to store the year and go on to setting the month and date.

For SID 2, hold down the SET button alone to start the process and
then use SET to store each setting.

Cheers,

Colin.