I HAVE the timer if I want to use it, but I don't need it. The instructions mention using it for a sort of "absolutely latest" time to close, or to delay opening when dawn comes around 4:45 a.m., wherever that occurs. Like, in Alaska, I guess, when the sun never sets for months, or only rises for a few hours in the winter.
My coop is under deciduous trees, but the sensor "reads" filtered light just fine. The door closes about 30 minutes AFTER the last chicken goes inside (except one night, when my usually last in hen missed it). She spent the night under the coop where I couldn't reach her, and never, ever missed the automatic door closing time again. My ducks often stay out, and they will occasionally miss it, but they have an alternate sleeping site. For the most part, unless they're swimming in their pool, they do make it into the coop before the door closes.