Why Now?

Chirp...chirp...chirp. "Did you hear that?" John asked at 10:30 last night. "No, what?" "Just wait, you'll hear it."

Chirp. It can't be a bird at this time of night. What is it? "The smoke alarm," was my response.

Of course, we were both sound asleep. But there was no choice but to get up and do something about it. I turned on lights. John went outside and got the ladder. When he climbed up and twisted it to take it down, there were wires attached, going into the ceiling. We both groaned. Years ago, the hard-wired alarm in our Colorado house started chirping. We couldn't get it to stop. We took it off the wall and disconnected it. Still, it chirped. We finally had to take a hammer to it and destroy the *** thing to find silence. Thankfully, this time John discovered a compartment for the battery. I went over to the RV and got the container of batteries. A new 9-volt one did the trick. Silence, sweet silence.
While the ladder was up and we were awake, he also changed the battery in the kitchen smoke detector. We didn't want another night wake-up chirp.

Twice every year, the public is advised to change the batteries in their smoke detectors--when day-light savings time begins and when it ends. The fall change-over is this coming Sunday.
Since Arizona doesn't observe daylight savings time, how did the smoke detector know it was time for a new a battery?