Another good deal on timekeeping is the navspark mini. I recently picked up a package deal on them and I am putting them in my computing hardware. They seem to be more accurate than my capacity to measure them, also they are very sensitive. Sensitive enough to work indoors with no issues, no loss of 3d fix, ever. The navspark has an internal RTC with battery backup and I am wondering if it would be possible to move my working DS3231 to some other hardware that needs it and use the navspark with battery backup as the RPI's primary RTC.
Tragically, PC hardware seems to lose all the accuracy of any add-on RTC that has better granularity than one second, as far as I can tell.