Visualise Date DIFF and Time DIFF

You have not indicated what aggregation you want - do you want the total duration by month, or the average duration for all events in that month? Only events that fall completely in that month, or partial events too (starting in the month or ending in the month)? What are you eventually doing with that data?

In general when you want to do datetime/duration math, don't use datetime/duration format. Use number format with days as the whole number part and the time as the fraction.  Only at the very last moment, when you display the result, convert it back to duration format.