How can I add "smart" functionality to an old thermostat?
Devices exist which are basically heaters with timers, which can be mounted under a traditional thermostat to bias it toward turning the heat off sooner. By juggling the two settings, this can be made to work somewhat like a setback thermostat.

Mostly obsolete, since not very efficient, but useful in cases where the owner forbids installing a real smart thermostat.