so what is "5 nines"?

a recent isp issued caused me to remember just exactly what “uptime” means...here's a little taste:

guarantee %	allowable downtime annually
99.9999	holy crap < 1 minute (six sigma)
99.999	5.2 minutes (famous "5 nines")
99.99	52.5 minutes
99.9	525.6 minutes (8.76 hours)
99	5256 minutes (87.6 hours)
if you are "lucky" to have a provider giving you a 90% guarantee, congrats, you could be down for an acceptable 36.5 days per year