System time gets set either from the GPS signal if not connected to the network or another NTP server if you are at boot. The satellite gives you a coarse time

System time gets set either from the GPS signal if not connected to the network or another NTP server if you are at boot. The satellite gives you a coarse time directly (accurate to milliseconds I think), plus a high precision frequency reference driven by the onboard atomic clock, which NTP uses to keep time. Once setup, this thing is supposedly capable of stability in the microsecond range, which is better than you'd get relying on an external server (dunno if true yet however). NTP can set your system time I believe. Lack of a RTC doesn't matter in this case because you wouldn't want one, they typically aren't stable enough. You're not supposed to turn it off very often anyway. If you were more serious about it than me you'd probably add a rubidium oscillator to serve as a secondary reference when GPS is offline and hook the whole thing up to a large battery, so that it never turns off. I don't care that much though, I'm just having fun with cheap toys.