diff -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1561,7 +1561,7 @@
  *   - timestamp:
  *   - text:
  */
-function template_preprocess_time(&$variables) {
+function template_preprocess_datetime(&$variables) {
   // Format the 'datetime' attribute based on the timestamp.
   // @see http://www.w3.org/TR/html5-author/the-time-element.html#attr-time-datetime
   if (!isset($variables['attributes']['datetime']) && isset($variables['timestamp'])) {
@@ -2580,9 +2580,9 @@
       'render element' => 'elements',
       'template' => 'region',
     ),
-    'datetime' => array(
+    'time' => array(
       'variables' => array('timestamp' => NULL, 'text' => NULL, 'attributes' => array(), 'html' => FALSE),
-      'template' => 'datetime',
+      'template' => 'time',
     ),
     'status_messages' => array(
       'variables' => array('display' => NULL),
@@ -3000,9 +3000,9 @@
       'render element' => 'elements',
       'template' => 'region',
     ),
-    'time' => array(
+    'datetime' => array(
       'variables' => array('timestamp' => NULL, 'text' => NULL, 'attributes' => array(), 'html' => FALSE),
-      'template' => 'time',
+      'template' => 'datetime',
     ),
     'status_messages' => array(
       'variables' => array('display' => NULL),
reverted:
--- b/core/modules/aggregator/aggregator.pages.inc
+++ a/core/modules/aggregator/aggregator.pages.inc
@@ -192,7 +192,7 @@
     ),
   ));
   $variables['age'] = array(
+    '#theme' => 'datetime',
-    '#theme' => 'time',
     '#attributes' => array(
       'datetime' => format_date($item->getPostedTime(), 'html_datetime', '', 'UTC'),
       'class' => array('feed-item-age',),
reverted:
--- b/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldFormatter/DatetimeDefaultFormatter.php
+++ a/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldFormatter/DatetimeDefaultFormatter.php
@@ -118,7 +118,7 @@
       // Display the date using theme datetime.
       // @todo How should RDFa attributes be added to this?
       $elements[$delta] = array(
+        '#theme' => 'datetime',
-        '#theme' => 'time',
         '#text' => $formatted_date,
         '#html' => FALSE,
         '#attributes' => array(
only in patch2:
unchanged:
--- a/core/modules/aggregator/aggregator.theme.inc
+++ b/core/modules/aggregator/aggregator.theme.inc
@@ -116,7 +116,7 @@ function template_preprocess_aggregator_summary_item(&$variables) {
     ),
   ));
   $variables['age'] = array(
-    '#theme' => 'datetime',
+    '#theme' => 'time',
     '#attributes' => array(
       'datetime' => format_date($item->getPostedTime(), 'html_datetime', '', 'UTC'),
       'class' => array('feed-item-age',),
only in patch2:
unchanged:
--- a/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldFormatter/DateTimeDefaultFormatter.php
+++ b/core/modules/datetime/lib/Drupal/datetime/Plugin/Field/FieldFormatter/DateTimeDefaultFormatter.php
@@ -123,7 +123,7 @@ public function viewElements(FieldItemListInterface $items) {
 
       // Display the date using theme datetime.
       $elements[$delta] = array(
-        '#theme' => 'datetime',
+        '#theme' => 'time',
         '#text' => $formatted_date,
         '#html' => FALSE,
         '#attributes' => array(