Thanks a lot for your post and great to hear that you are liking SSRS report.
Regarding time format, all the dates are stored in UTC in SQL Monitor repository. Not sure if you really want to change this to CST as then the report will show wrong time when clock change happens for day light saving. I have not used Reporting services a lot so not very sure but I think you would have to change the report directly.
I verified the duration, apologies it is typo on our end. The error is happening because of two extra zeros in conversion. So 19.04 is actually 1904 seconds which is 31 minutes and 44 seconds. We will upload the modified correct report on our website on Monday. But for now, I am emailing you the rdl file for your use. Thanks for pointing this out.