[SOLVED] Add a realtime clock

even with no wifi, net, ntp available, a thermostat - whether smart or not - should not lose its time and date.

You are right. This was an issue with HestiaPi Classic too but we realised that the non-internet connected devices are very few.
On its defense, you only need internet (NTP access) once and the time is preserved.
You can also set it manually once via SSH and it will preserve too:

sudo date --set “27 Jun 2014 1:17:00”

On the other hand, turn HestiaPi off, and its clock will stop ticking.
Adding an RTC is no big deal, its just that the circuit is getting crowded for the space available and air ventilation is already a headache.