TimestampDatetimeWidget::massageFormValues does not allow NULL value

Thank you for reporting this issue.

Next step would be to write a test case for it.

Steps to reproduce
create a timestamp field with the default value as empty
Edit the field and check the default value(we can see a value in the default value section)

If we are allowing the empty value then we need to change the description of the widget

The screenshot says leave blank to use time of form submission. So when the form is submitted it should still get a value and not be empty.

To me this is working by design.

What is the use case for wanting it to be empty?

thanks for the feedback ranjith_kumar_k_u and smustgrave

in my case, the field is used to define the unpublishing date of a node (another use case would be a birthday field)
leaving empty means the node won't be unpublished (automatically)

I understand that the widget was designed to work this way, so I would create a custom widget for my use case

For those landing here and wondering what a custom widget to get around this might look like, this kind of thing seems to do the trick:

<?php

namespace Drupal\my_module\Plugin\Field\FieldWidget;

use Drupal\Core\Datetime\Plugin\Field\FieldWidget\TimestampDatetimeWidget;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation of the 'nullable datetime timestamp' widget.
 *
 * @FieldWidget(
 *   id = "datetime_timestamp_nullable",
 *   label = @Translation("Nullable Datetime Timestamp"),
 *   field_types = {
 *     "timestamp",
 *   }
 * )
 */
class NullableTimestampDatetimeWidget extends TimestampDatetimeWidget {

  public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
    // Record any NULL values
    $null_values = [];
    foreach ($values as $delta => $item) {
      if (is_null($item['value'])) {
        $null_values[$delta] = $delta;
      }
    }

    // Let the core widget do what it wants
    $values = parent::massageFormValues($values, $form, $form_state);

    // Restore the null values
    foreach ($null_values as $delta) {
      $values[$delta]['value'] = NULL;
    }

    return $values;
  }

}