Well, in their defence, you can't really know how much the uptime is going to be affected by until the incident is resolved.
If it was a live counter then you'd see the uptime go up as the month progresses!
Basically time "down" / total time elapsed during $period. At the beginning of the month/quarter/period you'd be at 100% (if they are up at that moment). Or even simpler, just have it be a measurement of a rolling window X days in the past.
I've questioned status pages since.