In general, this is not advisable. The RTC was only intended to keep APPROXIMATE time in order to decrease search time when the receiver is restarted, it was not designed to be used as an accurate time source unless satellites are being tracked. When the receiver is actively tracking satellites, the time being reported by the receiver is extremely accurate as the RTC is slaved to GPS system time. However, if the receiver suffers a loss of lock due to satellite blockage or other system problems, the RTC will enter a free-running mode. The RTC will continue to run, but errors [...]
How is this possible? The key point here is that the satellite visibility number is a CALCULATED value, not the result of a real time activity being performed by the receiver. A receiver containing a valid almanac, time, and position can CALCULATE which satellites SHOULD BE visible (above the horizon) and will report these as VISIBLE. If a newly activated receiver has determined it's position but has yet to download an almanac, it will continue to report zero satellites visible until the almanac has been downloaded (approx. 15 minutes after the first satellite is acquired).