Medicare retirement date/benefit begin date

Does anyone know how to enter the Benefit Begin date for Medicare subscribers?  When an eligibility check is run in Procentive, we see the Medicare number and Benefit Begin date at the bottom of each eligibility check.  When adding Medicare to a client profile, there is no Benefit Begin date field to enter this in.  However, there is a "Retirement Date."  Is the Benefit Begin & Retirement Date the same thing?

Thanks!