It's a friendly conversation. v1.0 DOES use interrupts for print. You got it backwards. It does not miss a tick, but I believe that update to the millis() clock will be late. Still the total count will be exactly correct after many minutes of this problem happening. You can correct a RTC in code, the same way as the Uno clock. But what would be the point of this for timing events? They are both accurate after being calibrated in the code. A RTC clock is used if you need to know the time/date after powering down and up. There are many ways to achieve 1ppm accuracy. Software is easier than hardware if it fits your criteria. I'm trying to explain one way to correct the hardware limitations using code to calibrate the millis() clock. There aren't many ways to achieve this specific goal, once you have the constant calculated. But there are many ways of getting this number first. 99.99% would be good for this application, I agree. 10% is unacceptable for many applications.