It’s hard to tell without seeing your schematic. The firmware writes the current UTC time into RTC every time it gets a successful time sync from NTP server. The firmware maintains a system time. When the controller reboots, it will first retrieve the system time from the RTC. If it subsequently gets a successful NTP result, that will overwrite the system time (and writes that to RTC). The firmware does time keeping on ESP8266, which is pretty accurate, so it only needs to perform NTP occasionally to ensure the time is still accurate. The involvement of the RTC is pretty minimal here — it primarily covers the situation when the controller has rebooted but can’t access NTP yet (e.g. because the router hasn’t finished booting). Normally if the controller has NTP access (which is the common scenario), it will get time from that.