I just bought the Honeywell RPLS740B Econoswitch, a clever dawn/dusk switch that knows when the sun will be down year-round based on the latitude entered during setup. I’m using it to switch on exterior lights at dusk, saving me the trouble of frequently adjusting the set-point of the mechanical switch it replaces.
One feature I particularly appreciate is its use of a super-capacitor to keep time during power interruptions.
Every plug-in device with a clock should have a supercapacitor time backup. Some use batteries as clock backups, but many more — including expensive appliances like my various ovens — have no backup at all. Unlike batteries, capacitors have an unlimited service life: See this whitepaper on “Supercapacitors for RTC and Memory Backup.”