entityQuery: select entities created N days ago

$timestamp = \Drupal::time()->getCurrentTime() - (180 * 24 * 60 * 60);

$ids = \Drupal::entityQuery('activities')
 ->accessCheck(FALSE)
 ->condition('type', ['node_mention', 'comment_mention', 'comment_reply'], 'IN')
 ->condition('created', $timestamp, '<=')
 ->execute();