Not gonna do it.
You: Write some code that shows a millis() based clock pulling ahead or falling behind a timer/counter based clock.
Then we can examine your code for correctness, and run it ourselves.
There is nothing “real world” about this. A simulation would show the difference you assert would result.
We aren’t comparing to the passage of time, we are talking about the difference between two ways of counting off the system clock.
I am too sure you are wrong and too lazy to bother proving it, so until you do the (minimal) work to change my mind, I am filing you away.
FWIW it is way easier to just use a $3 RTC module that keeps time, real world time, way better.
a7