Is it possible to add a RTC to the Raspberry Pi that is in the controller? Thank you
yes, adding a RTC to Raspberry Pi should be easily feasible, there are many modules offered for this. But the ntp daemon which can be installed on Onefinity Controller which uses the NTP Protocol is more precise than a quartz-based rtc which may loose seconds per month. Of course NTP requires the Onefinity Controller to have network access to a NTP time server.
You may also equip the Onefinity Controller or a computer in your local net with a radio-based clock (here we use the DCF77 long-wave radio signal that receives its time from the atomic clock of the Physikalisch-Technische Bundesanstalt, a similar service may be present in your part of the world) which also works in case internet connection is absent.