How when AWS was down, we were not

I'm interested in how they measure that downtime. If you're down for 200 milliseconds, does that accumulate. How do you even measure that you're down for 200ms.

(For what it's worth, for some of my services, 200ms is certainly an impact, not as bad as 2 seconds out outage but still noticable and reportable)

I think a lot of web services talk about reliability in terms of uptime (e.g. down for less than 5 minutes a year) but in reality operate on failure ratios (less than 0.001% of request to our service fail).