Every place where an admin can set the next renewal date (the inline Next Renew Date editor, Reactivate/Uncancel, and the Update next renew day to: field in the Skip Day, Unskip Day and Cancel Day modals) refuses a date before today. The inline editor shows "Renewal date cannot be in the past."; the day modals and Reactivate/Uncancel silently keep the existing renewal date instead of applying the back-dated one. This protects against back-dating, which would make the subscription look overdue and cause it to be charged again on every scheduled run. Pick today or a future date instead; the buffer window will still start the appropriate number of days earlier.