How to calculate average time in hrs between start Date and End Date?

[#1]

Hi

I am new to QlikView, and trying to create a bar chart which shows average execution time in hrs for cases opened between Start Date and End Date.If a Case is completed then the end date will be populated by the app otherwise it will be blank (system time need to be taken into consideration)

X Axis --> would give the Open and complete case

y axis --> Average Time in hrs

Tried using the below formula for calculating the average time but no graph is displayed

if(isNull([End Date],

average (Interval(Time(TimeStamp#(Now(), 'D/M/YYYY hh:mm:ss TT')) - Time(TimeStamp#([Start Date], 'D/M/YYYY hh:mm:ss TT')), 'hh')),

average (Interval(Time(TimeStamp#([End Date], 'D/M/YYYY hh:mm:ss TT')) - Time(TimeStamp#([Start Date], 'D/M/YYYY hh:mm:ss TT')), 'hh')))

Need pointers in calculating the difference beteween Start date and end date in hrs

Regards

Sushil