How many years had Jr. John Conyers served in total?
Jr. John Conyers is an official_full_name; years served refers to SUM(SUBTRACT(END, start))
SELECT SUM(CAST(T2.END - T2.start AS DATE)) AS sum FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.official_full_name = 'John Conyers, Jr.'