Not sure if you are still using Flux or have moved on to l̶e̶s̶s̶ ̶f̶r̶u̶s̶t̶r̶a̶t̶i̶n̶g̶ ̶o̶p̶t̶i̶o̶n̶s̶ greener pastures, but this post explains how to compute the days in the selector time range. You could also modify it to calculate hours, minutes, etc.
