Cars and Respective Ride Lengths for a month - Date Select

select

cc.car_name as carname,
sum(TIMESTAMPDIFF(hour,CONVERT_TZ(r.start_at, '+00:00', '+8:00'),CONVERT_TZ(r.end_at, '+00:00', '+8:00'))) as ridelength

from reservations r

join cars c
on c.id = r.car_id

join car_classes cc
on cc.id = c.car_class_id


where CONVERT_TZ(r.start_at, '+00:00', '+8:00') >= '2019-1-1 00:00'
and CONVERT_TZ(r.start_at, '+00:00', '+8:00') <= '2019-1-31 00:00'
and r.state = 'completed'

group by cc.car_name