What if you store the StartTime and EndTime of the Procedure in the different entity as well, if you then use the between date functions to display the total time occurred in a string.
What if you create a microflow that will take the sum of the list items and sums up the allotted time and store that in a new attribute?
Did you use the Secondsbetween or something similar? function to calculate the allotted time? This will give you a decimal value if i'm correct. You can use this value to convert this to the amount of hours/minutes to show. It seems that the date time attribute is not really suitable to show a sum of hours (in my opinion)