UNO clock is not 10%. Standalone chip running on factory default clock is 10%.
Serial is not the only thing that uses interrupts and I think it's been established that 1.0 does not use interrupts.
If an IRQ is running when the millis update IRQ is supposed to be running then how doesn't it miss the tick?
When you have a 1ppm crystal and a more than 1ppm RTC then the technical term to look up is 'tolerance'. And if your 1ppm is due to software correction then why can't the RTC which should be as regular as it gets be corrected in the same way?
What part of "easy way" is turning into "only way to be accurate"?
What part of hardware limitations really means nothing can be done to correct them?
The more I see about the dyno the less I think it's going to need 1ppm timing anyway, it's in the 99.99%.
I am really ever so sorry I brought up problems that others have related on UNO timing. Sue me.