diff --git a/core/lib/Drupal/Core/Datetime/Element/Datelist.php b/core/lib/Drupal/Core/Datetime/Element/Datelist.php
index 59177e7..79cda1c 100644
--- a/core/lib/Drupal/Core/Datetime/Element/Datelist.php
+++ b/core/lib/Drupal/Core/Datetime/Element/Datelist.php
@@ -6,6 +6,7 @@
 use Drupal\Core\Datetime\DateHelper;
 use Drupal\Core\Datetime\DrupalDateTime;
 use Drupal\Core\Form\FormStateInterface;
+use DateTimeZone;
 
 /**
  * Provides a datelist element.
@@ -33,7 +34,7 @@ public function getInfo() {
       '#date_year_range' => '1900:2050',
       '#date_increment' => 1,
       '#date_date_callbacks' => [],
-      '#date_timezone' => '',
+      '#date_timezone' => drupal_get_user_timezone(),
     ];
   }
 
@@ -60,9 +61,8 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
           }
           unset($input['ampm']);
         }
-        $timezone = !empty($element['#date_timezone']) ? $element['#date_timezone'] : NULL;
         try {
-          $date = DrupalDateTime::createFromArray($input, $timezone);
+          $date = DrupalDateTime::createFromArray($input, $element['#date_timezone']);
         }
         catch (\Exception $e) {
           $form_state->setError($element, t('Selected combination of day and month is not valid.'));
@@ -77,6 +77,7 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
       if (!empty($element['#default_value'])) {
         $date = $element['#default_value'];
         if ($date instanceof DrupalDateTime && !$date->hasErrors()) {
+          $date->setTimezone(new DateTimeZone($element['#date_timezone']));
           static::incrementRound($date, $increment);
           foreach ($parts as $part) {
             switch ($part) {
@@ -183,17 +184,6 @@ public static function processDatelist(&$element, FormStateInterface $form_state
     // The value callback has populated the #value array.
     $date = !empty($element['#value']['object']) ? $element['#value']['object'] : NULL;
 
-    // Set a fallback timezone.
-    if ($date instanceof DrupalDateTime) {
-      $element['#date_timezone'] = $date->getTimezone()->getName();
-    }
-    elseif (!empty($element['#timezone'])) {
-      $element['#date_timezone'] = $element['#date_timezone'];
-    }
-    else {
-      $element['#date_timezone'] = drupal_get_user_timezone();
-    }
-
     $element['#tree'] = TRUE;
 
     // Determine the order of the date elements.
diff --git a/core/lib/Drupal/Core/Datetime/Element/Datetime.php b/core/lib/Drupal/Core/Datetime/Element/Datetime.php
index d005475..5e7c192 100644
--- a/core/lib/Drupal/Core/Datetime/Element/Datetime.php
+++ b/core/lib/Drupal/Core/Datetime/Element/Datetime.php
@@ -6,6 +6,7 @@
 use Drupal\Core\Datetime\DrupalDateTime;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Datetime\Entity\DateFormat;
+use DateTimeZone;
 
 /**
  * Provides a datetime element.
@@ -60,7 +61,7 @@ public function getInfo() {
       '#date_time_callbacks' => [],
       '#date_year_range' => '1900:2050',
       '#date_increment' => 1,
-      '#date_timezone' => '',
+      '#date_timezone' => drupal_get_user_timezone(),
     ];
   }
 
@@ -73,7 +74,6 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
       $time_input  = $element['#date_time_element'] != 'none' && !empty($input['time']) ? $input['time'] : '';
       $date_format = $element['#date_date_element'] != 'none' ? static::getHtml5DateFormat($element) : '';
       $time_format = $element['#date_time_element'] != 'none' ? static::getHtml5TimeFormat($element) : '';
-      $timezone = !empty($element['#date_timezone']) ? $element['#date_timezone'] : NULL;
 
       // Seconds will be omitted in a post in case there's no entry.
       if (!empty($time_input) && strlen($time_input) == 5) {
@@ -83,7 +83,7 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
       try {
         $date_time_format = trim($date_format . ' ' . $time_format);
         $date_time_input = trim($date_input . ' ' . $time_input);
-        $date = DrupalDateTime::createFromFormat($date_time_format, $date_time_input, $timezone);
+        $date = DrupalDateTime::createFromFormat($date_time_format, $date_time_input, $element['#date_timezone']);
       }
       catch (\Exception $e) {
         $date = NULL;
@@ -97,6 +97,7 @@ public static function valueCallback(&$element, $input, FormStateInterface $form
     else {
       $date = $element['#default_value'];
       if ($date instanceof DrupalDateTime && !$date->hasErrors()) {
+        $date->setTimezone(new DateTimeZone($element['#date_timezone']));
         $input = [
           'date'   => $date->format($element['#date_date_format']),
           'time'   => $date->format($element['#date_time_format']),
@@ -217,14 +218,6 @@ public static function processDatetime(&$element, FormStateInterface $form_state
     // The value callback has populated the #value array.
     $date = !empty($element['#value']['object']) ? $element['#value']['object'] : NULL;
 
-    // Set a fallback timezone.
-    if ($date instanceof DrupalDateTime) {
-      $element['#date_timezone'] = $date->getTimezone()->getName();
-    }
-    elseif (empty($element['#timezone'])) {
-      $element['#date_timezone'] = drupal_get_user_timezone();
-    }
-
     $element['#tree'] = TRUE;
 
     if ($element['#date_date_element'] != 'none') {
diff --git a/core/tests/Drupal/KernelTests/Core/Datetime/Element/TimezoneTest.php b/core/tests/Drupal/KernelTests/Core/Datetime/Element/TimezoneTest.php
new file mode 100644
index 0000000..bdf2334
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Datetime/Element/TimezoneTest.php
@@ -0,0 +1,392 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Datetime\Element;
+
+use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
+use Drupal\Core\Datetime\Entity\DateFormat;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\FormBuilderInterface;
+use Drupal\Core\Form\FormInterface;
+use Drupal\Core\Form\FormState;
+use DateTimeZone;
+use Drupal\Core\Datetime\DrupalDateTime;
+
+/**
+ * Tests the timezone handling of datetime and datelist element types.
+ *
+ * A range of different permutations of #default_value and #date_timezone
+ * for an element are setup in a single form by the buildForm() method, and
+ * tested in various ways for both element types.
+ *
+ * @group Form
+ */
+class TimezoneTest extends EntityKernelTestBase implements FormInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['system'];
+
+  /**
+   * The date used in tests.
+   *
+   * @var \Drupal\Core\Datetime\DrupalDateTime
+   */
+  protected $date;
+
+  /**
+   * An array of timezones with labels denoting their use in the tests.
+   *
+   * @var array
+   */
+  protected $timezones = [
+    // UTC-12, no DST.
+    'zone A' => 'Pacific/Kwajalein',
+    // UTC-7, no DST.
+    'zone B' => 'America/Phoenix',
+    // UTC+5:30, no DST.
+    'user' => 'Asia/Kolkata',
+    'UTC' => 'UTC',
+  ];
+
+  /**
+   * The test date formatted in various formats and timezones.
+   *
+   * @var array
+   */
+  protected $formattedDates = [];
+
+  /**
+   * HTML date format pattern.
+   *
+   * @var string
+   */
+  protected $dateFormat;
+
+  /**
+   * HTML time format pattern.
+   *
+   * @var string
+   */
+  protected $timeFormat;
+
+  /**
+   * The element type that is being tested ('datetime' or 'datelist').
+   *
+   * @var string
+   */
+  protected $elementType;
+
+  /**
+   * The number of test elements on the form.
+   *
+   * @var int
+   */
+  protected $testConditions;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+
+    $form['test1'] = [
+      '#title' => 'No default date, #date_timezone present',
+      '#type' => $this->elementType,
+      '#default_value' => '',
+      '#date_timezone' => $this->timezones['zone A'],
+      '#test_expect_timezone' => 'zone A',
+    ];
+
+    $form['test2'] = [
+      '#title' => 'No default date, no #date_timezone',
+      '#type' => $this->elementType,
+      '#default_value' => '',
+      '#test_expect_timezone' => 'user',
+    ];
+
+    $form['test3'] = [
+      '#title' => 'Default date present with default timezone, #date_timezone same',
+      '#type' => $this->elementType,
+      '#default_value' => $this->date,
+      '#date_timezone' => $this->timezones['user'],
+      '#test_expect_timezone' => 'user',
+    ];
+
+    $form['test4'] = [
+      '#title' => 'Default date present with default timezone, #date_timezone different',
+      '#type' => $this->elementType,
+      '#default_value' => $this->date,
+      '#date_timezone' => $this->timezones['zone A'],
+      '#test_expect_timezone' => 'zone A',
+    ];
+
+    $form['test5'] = [
+      '#title' => 'Default date present with default timezone, no #date_timezone',
+      '#type' => $this->elementType,
+      '#default_value' => $this->date,
+      '#test_expect_timezone' => 'user',
+    ];
+
+    $dateWithwithTimeZoneA = clone $this->date;
+    $dateWithwithTimeZoneA->setTimezone(new DateTimeZone($this->timezones['zone A']));
+    $form['test6'] = [
+      '#title' => 'Default date present with unusual timezone, #date_timezone same',
+      '#type' => $this->elementType,
+      '#default_value' => $dateWithwithTimeZoneA,
+      '#date_timezone' => $this->timezones['zone A'],
+      '#test_expect_timezone' => 'zone A',
+    ];
+
+    $form['test7'] = [
+      '#title' => 'Default date present with unusual timezone, #date_timezone different',
+      '#type' => $this->elementType,
+      '#default_value' => $dateWithwithTimeZoneA,
+      '#date_timezone' => $this->timezones['zone B'],
+      '#test_expect_timezone' => 'zone B',
+    ];
+
+    $form['test8'] = [
+      '#title' => 'Default date present with unusual timezone, no #date_timezone',
+      '#type' => $this->elementType,
+      '#default_value' => $dateWithwithTimeZoneA,
+      '#test_expect_timezone' => 'user',
+    ];
+
+    $this->testConditions = 8;
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installConfig(['system']);
+
+    // Setup the background time zones.
+    $this->timezones['php initial'] = date_default_timezone_get();
+    $user = $this->createUser([], []);
+    $user->set('timezone', $this->timezones['user'])->save();
+    // This also set's PHP's assumed time.
+    \Drupal::currentUser()->setAccount($user);
+
+    // Set a reference date to use in tests.
+    $this->date = new DrupalDatetime('2000-01-01 12:00', NULL);
+
+    // Create arrays listing the dates and times of $this->date formatted
+    // according to the various timezones of $this->timezones.
+    $this->dateFormat = DateFormat::load('html_date')->getPattern();
+    $this->timeFormat = DateFormat::load('html_time')->getPattern();
+    $date = clone $this->date;
+    foreach ($this->timezones as $label => $timezone) {
+      $date->setTimezone(new DateTimeZone($timezone));
+      $this->formattedDates['date'][$label] = $date->format($this->dateFormat);
+      $this->formattedDates['time'][$label] = $date->format($this->timeFormat);
+      $this->formattedDates['day'][$label] = $date->format('j');
+      $this->formattedDates['month'][$label] = $date->format('n');
+      $this->formattedDates['year'][$label] = $date->format('Y');
+      $this->formattedDates['hour'][$label] = $date->format('G');
+      $this->formattedDates['minute'][$label] = $date->format('i');
+      $this->formattedDates['second'][$label] = $date->format('s');
+    }
+
+    // Validate the timezone setup.
+    $this->assertEquals($this->timezones['user'], drupal_get_user_timezone(), "Subsequent tests assume specific value for drupal_get_user_timezone().");
+    $this->assertEquals(drupal_get_user_timezone(), date_default_timezone_get(), "Subsequent tests may assume PHP's time is set to Drupal user's time zone.");
+    $this->assertEquals(drupal_get_user_timezone(), $this->date->getTimezone()
+      ->getName(), "Subsequent tests assume DrupalDateTime objects default to Drupal user time zone if none specified");
+  }
+
+  /**
+   * Tests datetime elements interpret their times correctly when saving.
+   *
+   * Initial times are inevitably presented to the user using using a timezone,
+   * and so the time must be interpreted using the same timezone when it is
+   * time to save the form, otherwise stored times may be changed without the
+   * user changing the element's values.
+   */
+  public function testDatetimeElementTimesUnderstoodCorrectly() {
+    $this->assertTimesUnderstoodCorrectly('datetime', ['date', 'time']);
+  }
+
+  /**
+   * Tests datelist elements interpret their times correctly when saving.
+   *
+   * See testDatetimeElementTimesUnderstoodCorrectly() for more explanation.
+   */
+  public function testDatelistElementTimesUnderstoodCorrectly() {
+    $this->assertTimesUnderstoodCorrectly('datelist', [
+      'day',
+      'month',
+      'year',
+      'hour',
+      'minute',
+      'second',
+    ]);
+  }
+
+  /**
+   * On datetime elements test #date_timezone after ::processDatetime.
+   *
+   * The element's render array has a #date_timezone value that should
+   * accurately reflect the timezone that will be used to interpret times
+   * entered through the element.
+   */
+  public function testDatetimeTimezonePropertyProcessed() {
+    $this->assertDateTimezonePropertyProcessed('datetime');
+  }
+
+  /**
+   * On datelist elements test #date_timezone after ::processDatetime.
+   *
+   * See testDatetimeTimezonePropertyProcessed() for more explanation.
+   */
+  public function testDatelistTimezonePropertyProcessed() {
+    $this->assertDateTimezonePropertyProcessed('datelist');
+  }
+
+  /**
+   * Asserts that elements interpret dates using the expected time zones.
+   *
+   * @param string $elementType
+   *   The element type to test.
+   * @param array $inputs
+   *   The names of the default input elements used by this element type.
+   */
+  protected function assertTimesUnderstoodCorrectly($elementType, array $inputs) {
+    $this->elementType = $elementType;
+
+    // Simulate the form being saved, with the user adding the date for any
+    // initially empty elements, but not changing other elements.
+    $form_state = new FormState();
+    $form_builder = $this->container->get('form_builder');
+    $form = $this->setupForm($form_state, $form_builder);
+    foreach ($form as $elementName => $element) {
+      if (is_array($element) && array_key_exists('#type', $element) && $element['#type'] === $this->elementType) {
+        if ($element['#default_value'] === '') {
+          $newValues = [];
+          // Build an array of new values for the initially empty elements,
+          // depending on the inputs required by the element type, and using
+          // the timezone that will be expected for that test element.
+          foreach ($inputs as $input) {
+            $newValues[$input] = $this->formattedDates[$input][$element['#test_expect_timezone']];
+          }
+          $form_state->setValue([$elementName], $newValues);
+        }
+      }
+    }
+    $form_builder->submitForm($this, $form_state);
+
+    // Examine the output of each test element.
+    $utc = new DateTimeZone('UTC');
+    $expectedDateUTC = clone $this->date;
+    $expectedDateUTC->setTimezone($utc)->format('Y-m-d H:i:s');
+    $wrongDates = [];
+    $wrongTimezones = [];
+    $rightDates = 0;
+    foreach ($form_state->getCompleteForm() as $elementName => $element) {
+      if (isset($element['#type']) && $element['#type'] === $this->elementType) {
+        $actualDate = $form_state->getValue($elementName);
+        $actualTimezone = array_search($actualDate->getTimezone()
+          ->getName(), $this->timezones);
+        $actualDateUTC = $actualDate->setTimezone($utc)->format('Y-m-d H:i:s');
+
+        // Check that $this->date has not anywhere been accidentally changed
+        // from its default timezone, invalidating the test logic.
+        $this->assertEquals(drupal_get_user_timezone(), $this->date->getTimezone()
+          ->getName(), "Test date still set to user timezone.");
+
+        // Build a list of cases where the result is not as expected.
+        // Check the time has been understood correctly.
+        if ($actualDate != $this->date) {
+          $wrongDates[$element['#title']] = $actualDateUTC;
+        }
+        else {
+          // Explicitly counting test passes prevents the test from seeming to
+          // pass just because the whole loop is being skipped.
+          $rightDates += 1;
+        }
+        // Check the correct timezone is set on the value object.
+        if ($element['#test_expect_timezone'] !== $actualTimezone) {
+          $wrongTimezones[$element['#title']] = [$element['#test_expect_timezone'], $actualTimezone];
+        }
+      }
+    }
+
+    $message = "On all elements the time should be understood correctly as " . $expectedDateUTC . ": \n" . print_r($wrongDates, TRUE);
+    $this->assertTrue(($rightDates === $this->testConditions), $message);
+    $message = "On all elements the correct timezone should be set on the value object: (expected, actual) \n" . print_r($wrongTimezones, TRUE);
+    $this->assertTrue((count($wrongTimezones) === 0), $message);
+  }
+
+  /**
+   * Asserts that elements set #date_timezone correctly.
+   *
+   * @param string $elementType
+   *   The element type to test.
+   */
+  public function assertDateTimezonePropertyProcessed($elementType) {
+    $this->elementType = $elementType;
+    // Simulate form being loaded and default values displayed to user.
+    $form_state = new FormState();
+    $form_builder = $this->container->get('form_builder');
+    $this->setupForm($form_state, $form_builder);
+
+    // Check the #date_timezone property on each processed test element.
+    $wrongTimezones = [];
+    foreach ($form_state->getCompleteForm() as $elementName => $element) {
+      if (isset($element['#type']) && $element['#type'] === $this->elementType) {
+        // Check the correct timezone is set on the value object.
+        $actualTimezone = array_search($element['#date_timezone'], $this->timezones);
+        if ($element['#test_expect_timezone'] !== $actualTimezone) {
+          $wrongTimezones[$element['#title']] = [
+            $element['#test_expect_timezone'],
+            $actualTimezone,
+          ];
+        }
+      }
+      $this->assertEquals($this->timezones['user'], drupal_get_user_timezone(), "Subsequent tests assume specific value for drupal_get_user_timezone().");
+      $message = "The correct timezone should be set on the processed datetime elements: (expected, actual) \n" . print_r($wrongTimezones, TRUE);
+      $this->assertTrue((count($wrongTimezones) === 0), $message);
+    }
+  }
+
+  /**
+   * Simulate form being loaded and default values displayed to user.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   A form_state object.
+   * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
+   *   A form_builder object.
+   */
+  protected function setupForm(FormStateInterface &$form_state, FormBuilderInterface &$form_builder) {
+    $form_id = $form_builder->getFormId($this, $form_state);
+    $form = $form_builder->retrieveForm($form_id, $form_state);
+    $form_state->setValidationEnforced();
+    $form_state->clearErrors();
+    $form_builder->prepareForm($form_id, $form, $form_state);
+    $form_builder->processForm($form_id, $form, $form_state);
+    return $form_builder->retrieveForm($form_id, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'test_datetime_elements';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+  }
+
+}