Why doesn't my date get rendered?
I found it-- the Date field can store dates or datetimes, and you have to change your code for each case.
When you add a Date field to an entity, you can choose Date (date) or Date and time (datetime) as the Date type.
More information in this blog post, but the basic idea is:
$node-&gt;set('field_date', date('Y-m-d', time()));
$node-&gt;set('field_datetime', date('Y-m-d\TH:i:s', time()));