What is the meaning of 99.9% guaranteed up-time?
Guaranteed up-time is a usual norm in providing web hosting. Almost every vendor of hosting services guarantee 99.9% up-time. Some even offer less. What it means to be in real terms. These are the calculations.
99.9% in hours means 61258.68 hours guaranteed up-time.
(calculated by this formula= 24*7*365*99.9/100)
61.32 hours permissible downtime in a year
(calculated by this formula= 24*7*365*0.1/100)
3679.2 minutes permissible downtime in a year
(calculated by this formula=24*7*365*60*0.1/100)
Therefore the average daily permissible down time is about ten minutes daily or 10.08 minutes to be exact. This is calculated by dividing 3679.2 with 365.
Obviously it will be slightly less in a leap year which would be 10.0524590164.
minutes per day.
The conclusion is that a downtime of thirty minutes every three days is already written in 99.9%. So next time your website is down, checkup with these calculation for the hosts are not bothered with your concern.
BTW if you have wordpress jet pack installed, it keeps you informed about downtime and uptime by email, every time a website is down and and again when it is up.
Sandeep Bhalla's Blog 
Good point as to what uptime actually means.
Web hosts do not always the ultimate control. If they have their own servers on site, then yes – but if they have servers in remote data centres then their service can be affected by hardware issues or DDOS attacks there.
We have not lost even one second on our VPS in several years, but the Dallas data centre where we are located has had issues that have affected our web host and others and thousands of websites simultaneously.
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Right point. It does differ from server to server.
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