CMOS backup battery inside Surface

ah ok, thank you! ...so it is quite normal that if the main battery is completely empty the system does not autonomously move forward the date and time (without sync with a NTP server).

This is actually very rare and potentially dangerous. Products with built in batteries have a "hot battery bus" that is always powered when the battery is connected. It manages the clock and power on standby modules. This should consume minimal power so even if the battery is so low that it refuses to power on, the hot bus should work properly for months. Discharging the battery too much causes irreversible damage to the battery, it may even trip out to block charging forever. So avoid leaving the device uncharged for a very long time.