datetime field default gets truncated
As the BaseFieldDefenitions are cached, this won't work I believe.

I think you would be better off by using SetDefaultValueCallback:

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21BaseFieldDefinition.php/function/BaseFieldDefinition%3A%3AsetDefaultValueCallback/8.2.x