Doesn’t work for me either. Of course, the calendar says “100% uptime” even as there’s an outage right below it: https://status.slack.com/calendar
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.