timer on shabbat

Question

the hot plate was on and i needed another half hour before it was going to turn off. am i permitted to move the clip that sets a new time for the timer to go off? (i.e. it is 10 AM and the timer is set to go off at 11 AM , and would prefer for it to go off at 11:30) and what about setting an earlier or later time for the timer to go on in the future? (i.e. it is 10 am and the timer is set for 11 AM , and would prefer for it to go on at 1 PM instead.)