No way for a field to dynamically set default value

Slightly better title, I think.
I don't think this is or should be unique to date fields, but I cannot think of another use case either.

In short, a date field needs to be able to dynamically set its default value to now() or some value relative to now().

IMO, this might even qualify as major, but I will let some one else make that call.

I had a need for this as well. My use case was an entity reference field where the default value needed to be populated by a nodequeue. I ended up using _field_attach_form and jump through multiple hoops to reset the field_state (with field_form_get_state) and got something working.

In general I can imagine a lot of use cases where you want the default value to be depending on a certain context.

I resorted to creating widgets (extending the base widgets), whose only concern is to set the default value dynamically according to contextual values, e.g. I have a widget that presets a default value to an entity name referenced by the logged in user, something like this:

/**
 * Define the {@link AcmeWidget} class.
 *
 * @FieldWidget(
 *   id = "acme_widget",
 *   label = @Translation("Acme Textfield"),
 *   field_types = {
 *     "string"
 *   }
 * )
 *
 * @package Drupal\acme\Plugin\Field\FieldWidget
 */
class AcmeWidget extends StringTextfieldWidget {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {

    // Call our parent but set the value using our default value function.
    $element = parent::formElement($items, $delta, $element, $form, $form_state);

    // Get the current user organization's name.
    $user = User::load(\Drupal::currentUser()->id());
    $organization = $user->get('field_organization')->entity->title->value;

    // Set the default value if a value hasn't been set.
    $element['value']['#default_value'] = $organization;

    return $element;
  }


}

In D7, this can work as follows:

hook_form_alter

$form['field_name']['und'][0]['value']['#default_value'] = array($value);

But this does not work in D8:

hook_form_alter

$form['field_name']['widget'][0]['value']['#default_value'] = $value;

And this, too, does not lead to any result:

unset($form['field_name']['widget'][0]['value']['#default_value']);

This issue may no longer be valid, as there are ways to set default value callbacks on field instances:

1. Using BaseFieldDefinition->setDefaultValueCallback()
2. Using hook_entity_bundle_field_info_alter():
   

     if ($entity_type->id() == 'my_entity' && $bundle == 'my_bundle' && !empty($fields['field_my_field'])) {
       $fields['field_my_field']->setDefaultValueCallback('my_default_field_callback');
     }
   

3. Directly editing the field instance's config file as described at https://chromatichq.com/blog/dynamic-default-and-allowed-values-list-fie..., for example updating field.field.my_entity.my_bundle.my_field to include
   default_value_callback: 'my_default_field_callback'

#12 is spot on! Also, field types now have a way to provide default values globally in their item list's applyDefaultValue() method.