Light timers are used by many during Shabbat. The person sets the timer to switch the light on or off during Shabbat. I see this as problem because even if you set the timer before Shabbat, you have intentionally caused the light to go on or off during Shabbat. In-other-words you knew the delayed effect would change the light during Shabbat. Second, if you’re considered an observant person, others who pass by your house and see the lights turned on or off might be left with the impression that you have flipped the light switch. Why not just leave the light on or off? As a related question, would motion detector lights be prohibited on Shabbat? These are the kind of lights, outside of the house, that turn on or off automatically when something or someone passes in front of it.
The Torah forbids us the perform acts of "Melacha" (i.e. certain creative activities) on Shabbat, but not to refrain from causing these acts to happen on Shabbat. For instance, one may put a pot of raw food on the fire (if certain conditions are fulfilled) even though cooking is a Melacha and the cooking will take place on Shabbat. It is the human activity on Shabbat which is prohibited, not the result. Another way of saying this is that the individual must keep the restrictions of Shabbat, but his utensils do not have to observe them. The clock is a utensil. Motion detector lights are caused to go on by the person on Shabbat, so care should be taken not to activate such a light. If you do not know such a light is on but only suspect it, it is not necessary to refrain from walking by a certain place, but if you know the light is on you should not walk there.