Is there a way to round a timestamp up or down to the nearest 30 minute interval?

To round up - take processed date, add to it 30 minutes minus precision (1 minute in case of minutes, 1 second in case of seconds and so on), then take some "base" date ('2021-01-01' in case of my test data), find difference in minutes then divide by 30 and multiply by 30 and add the result back to the base date basically rounding down the modified date. Rounding down will work the same way without adding 30 minutes:

WITH dataset(time) AS (
   VALUES 
  (timestamp '2021-08-23 17:29:59'),
  (timestamp '2021-08-23 17:30:00'),
  (timestamp '2021-08-23 17:30:01'),
  (timestamp '2021-08-23 17:31:01'),
  (timestamp '2021-08-23 17:59:59'),
  (timestamp '2021-08-23 18:00:00'),
  (timestamp '2021-08-23 18:00:01')
 ) 
 
SELECT date_add(
  'minute', 
  date_diff(
    'minute',
    timestamp '2021-01-01', -- "base" date
    (time + interval '30' minute - interval '1' second) -- remove the +/- interval parts to round down
  ) / 30 * 30, 
  timestamp '2021-01-01') -- "base" date
FROM dataset

Output: