diff --git a/core/modules/datetime/datetime.module b/core/modules/datetime/datetime.module
index 245d0c4..a925706 100644
--- a/core/modules/datetime/datetime.module
+++ b/core/modules/datetime/datetime.module
@@ -27,7 +27,7 @@
  *
  * @see https://www.drupal.org/node/2912980
  */
-const DATETIME_DATETIME_STORAGE_FORMAT = 'Y-m-d\TH:i:s';
+const DATETIME_DATETIME_STORAGE_FORMAT = 'Y-m-d\TH:i:s+00:00';
 
 /**
  * Defines the format that dates should be stored in.
diff --git a/core/modules/datetime/src/DateTimeComputed.php b/core/modules/datetime/src/DateTimeComputed.php
index 16f664b..f0ba5f1 100644
--- a/core/modules/datetime/src/DateTimeComputed.php
+++ b/core/modules/datetime/src/DateTimeComputed.php
@@ -3,11 +3,11 @@
 namespace Drupal\datetime;
 
 use Drupal\datetime\Plugin\Field\FieldType\DateTimeItem;
+use Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface;
 use Drupal\Core\Datetime\DrupalDateTime;
 use Drupal\Core\TypedData\DataDefinitionInterface;
 use Drupal\Core\TypedData\TypedDataInterface;
 use Drupal\Core\TypedData\TypedData;
-use Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface;
 
 /**
  * A computed property for dates of date time field items.
@@ -47,7 +47,16 @@ public function getValue() {
     $value = $item->{($this->definition->getSetting('date source'))};
 
     $datetime_type = $item->getFieldDefinition()->getSetting('datetime_type');
-    $storage_format = $datetime_type === DateTimeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
+    if ($item->getFieldDefinition()->getSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {
+      $storage_format = DateTimeItemInterface::DATE_STORAGE_FORMAT;
+    }
+    else {
+      // Strip off an explicit time zone offset from the format and replace
+      // with the time zone offset placeholder to allow the string to be parsed
+      // properly.
+      $storage_format = preg_replace('/\\\+\d\d\:\d\d$/', 'P', DateTimeItemInterface::DATETIME_STORAGE_FORMAT);
+    }
+
     try {
       $date = DrupalDateTime::createFromFormat($storage_format, $value, DateTimeItemInterface::STORAGE_TIMEZONE);
       if ($date instanceof DrupalDateTime && !$date->hasErrors()) {
diff --git a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeFormatterBase.php b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeFormatterBase.php
index a6680f6..1d41688 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeFormatterBase.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeFormatterBase.php
@@ -227,7 +227,7 @@ protected function buildDate(DrupalDateTime $date) {
    */
   protected function buildDateWithIsoAttribute(DrupalDateTime $date) {
     // Create the ISO date in Universal Time.
-    $iso_date = $date->format("Y-m-d\TH:i:s") . 'Z';
+    $iso_date = $date->format("Y-m-d\TH:i:sP", ['timezone' => 'UTC']);
 
     $this->setTimeZone($date);
 
diff --git a/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
index 3264069..6528192 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
@@ -69,7 +69,7 @@ public static function schema(FieldStorageDefinitionInterface $field_definition)
         'value' => [
           'description' => 'The date value.',
           'type' => 'varchar',
-          'length' => 20,
+          'length' => 25,
         ],
       ],
       'indexes' => [
diff --git a/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItemInterface.php b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItemInterface.php
index a9231c6..ae0343e 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItemInterface.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItemInterface.php
@@ -15,7 +15,7 @@
   /**
    * Defines the format that date and time should be stored in.
    */
-  const DATETIME_STORAGE_FORMAT = 'Y-m-d\TH:i:s';
+  const DATETIME_STORAGE_FORMAT = 'Y-m-d\TH:i:s\+00:00';
 
   /**
    * Defines the format that dates should be stored in.
diff --git a/core/modules/datetime/tests/src/Kernel/DateTimeItemTest.php b/core/modules/datetime/tests/src/Kernel/DateTimeItemTest.php
index 75f892a..dc145b1 100644
--- a/core/modules/datetime/tests/src/Kernel/DateTimeItemTest.php
+++ b/core/modules/datetime/tests/src/Kernel/DateTimeItemTest.php
@@ -69,7 +69,7 @@ public function testDateTime() {
 
     // Verify entity creation.
     $entity = EntityTest::create();
-    $value = '2014-01-01T20:00:00';
+    $value = '2014-01-01T20:00:00+00:00';
     $entity->field_datetime = $value;
     $entity->name->value = $this->randomMachineName();
     $this->entityValidateAndSave($entity);
@@ -84,7 +84,7 @@ public function testDateTime() {
     $this->assertEquals(DateTimeItemInterface::STORAGE_TIMEZONE, $entity->field_datetime->date->getTimeZone()->getName());
 
     // Verify changing the date value.
-    $new_value = '2016-11-04T00:21:00';
+    $new_value = '2016-11-04T00:21:00+00:00';
     $entity->field_datetime->value = $new_value;
     $this->assertEqual($entity->field_datetime->value, $new_value);
     $this->assertEquals(DateTimeItemInterface::STORAGE_TIMEZONE, $entity->field_datetime->date->getTimeZone()->getName());
@@ -164,7 +164,7 @@ public function testSetValue() {
 
     // Test DateTimeItem::setValue() using string.
     $entity = EntityTest::create();
-    $value = '2014-01-01T20:00:00';
+    $value = '2014-01-01T20:00:00+00:00';
     $entity->get('field_datetime')->set(0, $value);
     $this->entityValidateAndSave($entity);
     // Load the entity and ensure the field was saved correctly.
@@ -175,7 +175,7 @@ public function testSetValue() {
 
     // Test DateTimeItem::setValue() using property array.
     $entity = EntityTest::create();
-    $value = '2014-01-01T20:00:00';
+    $value = '2014-01-01T20:00:00+00:00';
     $entity->set('field_datetime', $value);
     $this->entityValidateAndSave($entity);
     // Load the entity and ensure the field was saved correctly.
@@ -220,7 +220,7 @@ public function testSetValueProperty() {
     $this->fieldStorage->setSetting('datetime_type', DateTimeItem::DATETIME_TYPE_DATETIME);
     $this->fieldStorage->save();
     $entity = EntityTest::create();
-    $value = '2014-01-01T20:00:00';
+    $value = '2014-01-01T20:00:00+00:00';
 
     $entity->set('field_datetime', $value);
     $this->entityValidateAndSave($entity);
@@ -269,10 +269,10 @@ public function datetimeValidationProvider() {
     return [
       // Valid ISO 8601 dates, but unsupported by DateTimeItem.
       ['2014-01-01T20:00:00Z'],
-      ['2014-01-01T20:00:00+04:00'],
       ['2014-01-01T20:00:00+0400'],
       ['2014-01-01T20:00:00+04'],
       ['2014-01-01T20:00:00.123'],
+      ['2014-01-01T20:00:00'],
       ['2014-01-01T200000'],
       ['2014-01-01T2000'],
       ['2014-01-01T20'],