UPDATE timetable
SET eta = (
  CASE
    WHEN (trips.starttime::int / 100) + (destination.zuluoffset * -1 ) < 24 THEN ((trips.starttime::int / 100) + (destination.zuluoffset * -1 )) * 100
    WHEN (trips.starttime::int / 100) + (destination.zuluoffset * -1 ) > 23 THEN ((trips.starttime::int / 100) + (destination.zuluoffset * -1 ) - 24) * 100
  END )
FROM
  destination,
  trips
WHERE
  timetable.tripsid = trips.id;

UPDATE timetable
SET eta = lpad(eta, 4, '0');

SET eta = lpad((CASE statement here), 4, '0')

ERROR:  function lpad(numeric, integer, unknown) does not exist
LINE 3: SET eta = lpad((
                  ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.