The master node in a PTP network probably takes its time from a
precise external time source, like GPS. The GPS provides a 1 PPS
directly to the PTP clock hardware, which latches the PTP hardware
clock time on the PPS edge. This provides one sample as input to a
clock servo (in the PTPd) that, in turn, regulates the PTP clock
hardware.

A PTP clock is TAI, Unix time is UTC.

This is the core issue and source of misunderstanding, in my view. The
fact of the matter is, the current generation of computers has
multiple clocks, and these are usually unsynchronized. I think we
should not try too hard to cover up or work around this. It is a fact
of life.

In this case I don't think you can. Their divergence is rather difficult
to handle unless you have a GPS to hand.

But all this talk of "PTP this" and "PTP that" is not helpful. Any
interface for additional time sources should be generic with PTP being
one use case.

Alan