Since many people have asked about this, let us discuss this topic briefly without delving into the halachic intricacies.
We have already explained in the past that one may set a timer before Shabbat to turn on/off any electrical appliance, such as lights, air conditioners, and electric hotplates on Shabbat.
Digital timers which require some sort of electric function to change its current status (or a mechanical timer which must be completely stopped in order to change its status) may not be changed on Shabbat at all. Our entire discussion relates only to regular timers which are surrounded by clips or pins which control whether or not the appliance will go on or off at a certain time.
Extending the “On” Status
Let us now discuss when it is permissible or forbidden to change the status of a timer on Shabbat:
When the lights or appliance is on and one wishes to extend this status so that it remains on longer than it would if one left the timer as is, for instance, if the timer is set to turn the lights off at ten o’clock and one wishes to extend it to eleven o’ clock, one may move, push, or pull the pins in order to delay the lights turning off.
Making it Turn Off Earlier
One should not change the current status and set the timer to have the electricity turn off earlier. (However, if this is being done to turn the lights off earlier for a sick individual who is bothered by the light, this would be permissible. See Yalkut Yosef and Halacha Berura Chapter 277 for an in-depth explanation.)
Extending the “Off” Status
If the electricity is currently off and one wishes to prolong this status until a later time, for instance, if the light is supposed to turn on at eleven o’clock and one wishes for it to turn on only at twelve o’clock, this too is permissible by moving the pins accordingly in order to extend the current “off” status.
Making it Turn On Earlier
However, one should not change the current “off” status in order to have the electricity turn on earlier. (Some permit this when this is being done for the purpose of a Mitzvah.)
Summary: One may change the pins on a timer to extend its current status, i.e., for the electricity to remain on or off for a longer time, however, one may not change the current status of the timer by making it turn on or off earlier than it would have based on its setting at the onset of Shabbat.