Write SQL query to solve given problem: Calculate the average duration travelled by subscribers that both started and ended their trip in Mountain View City Hall and indicate the date when the station was first installed.. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT AVG(T1.duration), T2.installation_date FROM trip AS T1 INNER JOIN station AS T2 ON T2.name = T1.start_station_name WHERE T1.start_station_name = 'Mountain View City Hall' AND T1.subscription_type = 'Subscriber' AND T1.end_station_name = 'Mountain View City Hall'