Total Ride length (in hrs) for a certain time Period

select

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

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'