Add support for relative default dates in Date field

Looks a nice improvement, but needs reroll.

Code looks good to me.

Tested it on simplytest.me by editing the article content type, adding a field field_date with relative date, and default to "now". Default date when creating an article is today.

Edited the field, set the default to "+1 month". When creating an article, the default it is correctly set to one month from now.

+++ b/core/modules/datetime/src/Tests/DateTimeFieldTest.php
@@ -325,15 +326,49 @@ function testDefaultValue() {
+      'default_value_input[default_date]' => $this->randomString(),

Are you sure that in all cases this will generate an invalid relative date? This does not look like a good usage of random string.

1. +++ b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeFieldItemList.php
   @@ -32,13 +37,27 @@ public function defaultValuesForm(array &$form, FormStateInterface $form_state)
   +            static::DEFAULT_VALUE_NOW => t('The current date'),
   +            static::DEFAULT_VALUE_CUSTOM => t('Relative value'),
   

   Maybe use 'Current date' and 'Relative date' ?

2. +++ b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeFieldItemList.php
   @@ -32,13 +37,27 @@ public function defaultValuesForm(array &$form, FormStateInterface $form_state)
   +          '#description' => t("Describe a time by reference to the current day, like '+90 days' (90 days from the day the field is created) or '+1 Saturday' (the next Saturday). See !strtotime for more details.", array('!strtotime' => l(t('strtotime'), 'http://www.php.net/manual/en/function.strtotime.php'))),
   

   I'm not sure strtotime needs to be translated.

3. +++ b/core/modules/datetime/src/Tests/DateTimeFieldTest.php
   @@ -325,15 +326,49 @@ function testDefaultValue() {
   +    // Create a new node to check that datetime field default value is today.
   +    $new_node = entity_create('node', array('type' => 'date_content'));
   +    $expected_date = new DrupalDateTime('+90 days', DATETIME_STORAGE_TIMEZONE);
   +    $this->assertEqual($new_node->get($field_storage->name)->offsetGet(0)->value, $expected_date->format(DATETIME_DATE_STORAGE_FORMAT));
   

   Comment says 'today' but the value is +90 days ?

Also, the schema probably needs to be updated as well no ?

Great, looks good to me now!

Committed 4192e7d and pushed to 8.0.x. Thanks!

Relative dates work very well, many thanks! There seems to be some issues when time is involved.
Please have a look at the issue below:

https://www.drupal.org/project/drupal/issues/3169876