I am having the same problem, but a utag_data solution would require a dev cycle. Due to EOY time constraints I will (for now anyway) have to leverage the user-side start of session. I think I follow your suggestion, but I would appreciate a validation of my understanding.
-In the data layer, create a first party cookie, called utag_main_event_date
-associate my utag_main_event_date to my Webtrends tag
-Create a JS Extension. The extension should call the ses_id parameter. Convert the ses_id to GMT and add 8 (we are PST).
Thanks for the help!
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