reverted:
--- b/core/modules/datetime/datetime.module
+++ a/core/modules/datetime/datetime.module
@@ -46,11 +46,11 @@
  * Sets a consistent time on a date without time.
  *
  * The default time for a date without time can be anything, so long as it is
+ * consistently applied. If we use noon, dates in most timezones will have the
+ * same value for in both the local timezone and UTC.
- * consistently applied. Since no timezone conversion is done for date-only
- * fields, simply use 00:00:00 to avoid confusion.
  *
  * @param $date
  */
 function datetime_date_default_time($date) {
+  $date->setTime(12, 0, 0);
-  $date->setTime(0, 0, 0);
 }
diff -u b/core/modules/datetime/src/Tests/DateTimeFieldTest.php b/core/modules/datetime/src/Tests/DateTimeFieldTest.php
--- b/core/modules/datetime/src/Tests/DateTimeFieldTest.php
+++ b/core/modules/datetime/src/Tests/DateTimeFieldTest.php
@@ -188,6 +188,10 @@
       $options = array(
         'format_type' => array('short', 'medium', 'long'),
       );
+      // Formats that display a time component for date-only fields will display
+      // the default time, so that is applied before calculating the expected
+      // value.
+      datetime_date_default_time($date);
       foreach ($options as $setting => $values) {
         foreach ($values as $new_value) {
           // Update the entity display settings.