The calendar is straight forward unless it’s passed to another more concise external calendar. Perhaps what you’re seeking is a Reports extension. Trying reviewing some the available extensions for generating reports on user activity.
I have tried several more times with the internal Report capability, but Calendar isn’t an option that can be filtered. More so, the internal Report generator wasn’t designed with finesse. Off hand, I still couldn’t find a proper filter because filtering by Event is more defined that just reviewing user activity. Most likely a sort by task, event or some other filter will is suitable. However, I do suggest grabbing a Reporting extension or module for the exact measures you require.