Note that the version of the code posted above uses exttimer_millis()
which is my homebrew version of millis() that doesn't do any better than the library version.
But regular millis() can be substituted everywhere (and originally was) and code functionality is not affected.
As you can also see in the main loop I am just printing out millis() and this value is drifting with respect to time-stamping from RealTerm.
The drift is around 5 seconds over a 1439-second interval. This is huge (5 seconds error is 226 arc-seconds of angular displacement of the telescope mount, I need to maintain at least 2 arc-seconds over that time period).