One of the things most startling about Drupal 8 is the lack of documentation or false documentation on many simple everyday tasks. I went looking for how to set a (date) field on an entity form when another entity was selected from an entity reference autocomplete field on that form.
It took me a while to track this down so I wanted to post it somewhere. In the past I would have posted to api.drupal.org but it seems as though that has been rendered useless in D8 as each new release of D8 (8.3, 8.4, etc) hides the previous comments.
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