Abstract: The timer is adapted to be attached to a toggle light switch and comprises a body having a trip slide mounted in the body, with the slide having means for receiving and engaging the switch handle for tripping same to operate the lights. Spring means is provided in the body engageable with the slide for urging the slide to turn the lights off. When the slide is moved to compress the spring the lights are turned on. With the lights on, a latch carried by the slide engages the body to maintain the slide latched with the spring compressed. Fluid responsive timing means, located within the body, is engageable with the latch to operate and release the latch from engagement with the body after expiration of a predetermined time interval. Thereafter the compressed spring means moves the slide in a direction to trip the switch handle, thus turning the lights off.