Erlang's 99.9999999% (nine nines) reliability

Question

Erlang was reported to have been used in production systems for over 20 years with an uptime percentage of 99.9999999%.

I did the math as the following:

20*365.25*24*60*60*(1 - 0.999999999) == 0.631 s

That means the system only has less than one second of downtime during the period of 20 years. I am not trying to challenge the validity of this, I am just curious about how we can shut down a system (on purpose or by accident) for only 0.631 second. Could anyone who are familiar with large software system explain this to us? Thank you.

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Does anyone know how to calculate the downtime of a service over a cluster of processing units (or machines)?