shabbat clock/timer

Question

the only problem i have with the shabbat clock is that i feel that if i can move the trigger of the timer.. then i can essentially set the light to go on or off on shabbat. i.e. if it is 11 AM and i am having company at 11:30, then i can go over to the timer and place an on trigger to turn the light on at 11:30. then as they walk out at 2 PM (long visit) i can move another one to turn the lights off. is this permissible?? if it is why can i do this . what is the difference between turning the light on by flicking a switch or by using a "switch" that has a 15 minute delay (i.e. the timer)??