> This reminds me, one interesting thing I've noticed over the years is that early PC's RTCs were pretty accurate, but the most recent ones, as in the past few years, are horrible

Insofar as sensitivity to thermal conditions is an issue, early PCs tended to be big, airy, ventilated boxes and have less bits that would ramp up to high temperature under load but drop down in other operating regimes. So that may be a factor.