We still use that method as a backup to GPS, in case it fails, even today. All you have to do in preparation is to have a decent watch, and a book about navigation. Write down the date and the error compared to GPS every day at the same time. After awhile you don't have to do it anymore, maybe once a month. Now you have an accurate time from a $5 watch. It's the same concept with our 10% Uno clock. I tested 6 differrent Uno's, all well within 100ppm, before calibration.
Nick, my results agree with your theory.
Regardless of how interrupts effect millis(), there are no interrupts with the old IDE during Serial.print
I disagree, My DS1307 RTC is NOT MORE ACCURATE.
It will not miss a tick if it's coincidentally processing another interrupt during that tick. So long as it's not doing it during 2 in row, and the whole time in between. What Nick said.
Again, Serial.print does not cause an interrupt.
To put it into terms that matter, my Uno loses 1 second every 11 days when it is not synced to GPS, by calibrating in code. My PC loses 30 seconds. Not many RTC's can beat that.