diff --git a/core/modules/datetime/src/Plugin/Field/FieldWidget/DateRangeWidgetBase.php b/core/modules/datetime/src/Plugin/Field/FieldWidget/DateRangeWidgetBase.php
index d694c28..ed535d1 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldWidget/DateRangeWidgetBase.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldWidget/DateRangeWidgetBase.php
@@ -25,6 +25,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
 
     $element['#theme_wrappers'][] = 'datetime_wrapper';
     $element['#attributes']['class'][] = 'container-inline';
+    $element['#element_validate'][] = [$this, 'validateStartEnd'];
 
     $element['start_value'] = array(
       '#title' => $this->t('Start'),
@@ -127,4 +128,24 @@ public function massageFormValues(array $values, array $form, FormStateInterface
     return $values;
   }
 
+  /**
+   * Validates that the start <= the end date.
+   */
+  public function validateStartEnd($element, FormStateInterface $form_state) {
+    $start_date = $element['start_value']['#value']['object'];
+    $end_date = $element['end_value']['#value']['object'];
+
+    if ($start_date instanceof DrupalDateTime && $end_date instanceof DrupalDateTime) {
+      /** @var \Drupal\Core\Datetime\DrupalDateTime $start_date */
+      /** @var \Drupal\Core\Datetime\DrupalDateTime $end_date */
+
+      if ($start_date->format('U') !== $end_date->format('U')) {
+        $interval = $start_date->diff($end_date);
+        if ($interval->invert === 1) {
+          $form_state->setError($element, $this->t('Start date should be equal to, or before, end date'));
+        }
+      }
+    }
+  }
+
 }