As per our Terms of Service:
"[HOST.NAME] reserves the right to interrupt service no more than five (5) percent of the year for maintenance and updates to our site. This service interruption does not include power outages, service loss or any other reasons as listed in Section 21 Force Majeure."
Basically, we guarantee 95% uptime except for things outside of our control like earthquakes, our ISP has problems, etc. Our goal, however, is to have 99.5% uptime. Last year, our uptime was 99.6%.