For years the clock on any control would stop when the power stopped and then would start up again when the power came back on.
Then some of the newer ones or with in the last 15 years would have a 9volt that would hold the clock running even if the power went out, but even that was not good enough.
I am not sure if the new Clack Digital or the newer Fleck SXT control will keep the clock running if the power goes out.
You could buy one of the computer back up battery systems to power your softener if the main power goes out.