DIT HIER
SELECT date_trunc('hour', read_on_tz) + date_part('minute', read_on_tz)::int / 1 * interval '30' || '  min' AS read_on_hour,
       max(value), min(value)
FROM
  (SELECT *, ((read_on AT TIME ZONE 'UTC') AT TIME ZONE 'EUROPE/BRUSSELS') AS read_on_tz
   FROM gas_datapoints AS foo) AS timezoned
WHERE read_on_tz > '2020-12-22 00:00:00'
GROUP BY read_on_hour
ORDER BY read_on_hour desc;