I’ve noticed that there is a quite big drift with the System.DateTime method : after roughly 40 min, there are 6 sec more in System.DateTime when compared to Rtc 
Second line is RTC and third line is System.DateTime. “dt” is time drift, “el” is elapsed time since the program is running.
I understand that system time may not be as accurate as Rtc, but I think that this is a too big difference in such a little time.
Is there anything I can do on my side to try to be more accurate in case Rtc is or becomes not available ?
Edit : after 1 hour, there are 9 sec between the two dates.