We don't specifically track that, no. The worst one was when we went down for (IIRC) a couple days because of a disk failure, I think in Jan 2014. It was after that that we added a failover box.

HN goes down when we restart the server process, usually as part of updating the code - but only for a few seconds. The message "Restarting the server. Shouldn't take long." displays when that is happening.

There are also, to my exasperation, still moments of brownout during certain traffic spikes or moments of obscure resource contention. But these are at least rarer than they used to be.