The most reliable measure of true uptime would be transaction monitoring, e. g. simulating a login into all of these web apps. It also catches those issues were the server itself is up but the site does not work for some other reasons (database issue, javascript issue,...).
Or do you know that all your downtime was "total downtime", i. e. server not reachable at all?
Or do you know that all your downtime was "total downtime", i. e. server not reachable at all?