Calculate the total on events with two time conditions
Thanks to @mustaccio's pointer above. Here's how I managed to get it to work using INTERSECT DISTINCT.
SELECT COUNT(*) FROM (
  SELECT DISTINCT
    event_id
  FROM
    `foo.bar.blah`
  WHERE
    DATE(timestamp) = current_date()

  INTERSECT DISTINCT

  SELECT DISTINCT
    event_id
  FROM
    `foo.bar.blah`
  WHERE
    (DATE(timestamp) BETWEEN DATE_SUB(current_date(), INTERVAL 30 DAY) AND DATE_SUB(current_date(), INTERVAL 10 DAY))
)

The two dates in the last SELECT needs to be adjusted according to the criterion of course.