Do views filter plugins not apply to base field definitions?
Looks like this is a known major issue. Here's the main "parent" issue where most of the work is going down: https://www.drupal.org/node/2337515

I think, by default, there is some basic views integration for all entities, but it's not that robust.

As a workaround, I was able to implement hook_views_data_alter to tell views to use the 'datetime' filter plugin instead of the default 'string' plugin it had.

function MY_MODULE_views_data_alter(array &amp;$data) {
  $data['my_entity_type']['start_date']['filter']['id'] = 'datetime';
}