Write SQL query to solve given problem: What is the opening time of the active businesses in Glendale that have a medium review count.. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT DISTINCT T2.opening_time FROM Business AS T1 INNER JOIN Business_Hours AS T2 ON T1.business_id = T2.business_id INNER JOIN Days AS T3 ON T2.day_id = T3.day_id WHERE T1.city = 'Glendale' AND T1.review_count = 'Medium' AND T1.active = 'true'