Some clarifying questions about your requirements:
In a 30-day period, there's no way to tell that node A was unmanaged for three different time spans of 30 minutes, 60 minutes, and 15 minutes for a total of 105 minutes. I assume that's what you're trying to get in that column. Is that right?
Update: As I submitted the above it occurs to me that it could be calculated by looking for events and getting the spans between them but that would be difficult. Let's clarify that's what you're looking for before going down that road.
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