This is one instance where 99.999% reliability doesn't really cut it.
It says something interesting about how think about "high reliability." What it really means is "must never, ever fail" which kind of makes "% reliability" a sort of strange euphemism, I think.
If your timeline is only one day, 99.999% reliability is less than a second of downtime.
Of course this is taking op very literally
EDIT: I checked: google "0.001% * 1 year" and it tells you 5.259 minutes, so you were off due to dropping the % sign. That changes things dramatically: beyond the 100x difference, there is very high likelihood that someone would check back 2 minutes later (especially if they are waiting all year), so 5 minutes of downtime (spread over a whole day) likely objectively results in much lower lost sales than your calculation implied! For future envelope calculations don't forget that a % sign is exactly moving the decimal two more places! 55.2% is exactly .552 and so forth!
What I do is, have an active-active system in a data center, and with another similar system in another DC.
Having said that, your average online stock brokers do the same thing.