Going a year without any sort of outage is. For example, if you're just serving from one place, any network outage between the users and that place will be an outage -- seamless failover is pointlessly hard and very few shops do this for their internally-hosted services.
Depends how you define a network outage. One of my transatlantic circuits has been a bit ropey - had a 2 second outage on it a couple of nights ago, does that count as a network outage?
If a user has a local network problem, that's not a loss in the service. If multiple users do that's an issue. My home broadband went out for 3 minutes at 23:33:43 GMT on May 13th, but then I wouldn't run a service needing 24/7 on my home broadband.
My frickin nasdrive has a better uptime than slack.
Having a process running for 1511 days does not mean a service has 1511 days of uptime -- there are a ton of places stuff can go wrong.