Use the date on which you watched the first video lesson of the upgrade you received. If you don't remember when that was, then simply use the date on which you received the upgrade (check your email inbox to see when you got it.)
i purchased my PM Prepcast last March 1, 2013 but on July 8, 2013 i received an Upgrade align from PMBOK 5. now i will apply for PMP exam which date i will put in "Course Start Date" for my 35 contact hours.
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