"9 nines works out to 31 milliseconds of downtime per year. That's smaller than the acceptable drift of an atomic clock over the same period. You couldn't measure anything to that accuracy if you tried."
Not to be pedantic, but are you using the right unit of time? Atomic clocks aren't supposed to lose more than 1 second of accuracy over a million years... maybe you meant micro/nano/picoseconds? 31millisecond drift(assuming it goes one way) per day = 1 sec error over approximately 300 days...
Edit: Bah, the parent comment is gone. I'm leaving mine up anyway in case I'm wrong in my assessment.
