I figure it’s gotta be in there somewhere, because the console can display it when you use the “View action info” on the reported computer of an action.
Looks like @leewei made a Web Report for this some time back, but I don’t know whether it’s still valid in 9.5 - https://www.ibm.com/developerworks/community/forums/html/topic?id=77777777-0000-0000-0000-000014744532
Ah! and most importantly, it’s not exposed in Session Relevance, but is exposed in REST (- and thanks again Lee Wei) Actual action start and end time