Issue #2110777 by larowlan, herom: Fixed Cannot edit a date format after creating it.

core/modules/system/lib/Drupal/system/Form/DateFormatFormBase.php

@@ -197,7 +197,7 @@ public function validate(array $form, array &$form_state) {
     // name, check to see if the provided pattern exists.
     $pattern = trim($form_state['values']['date_format_pattern']);
     foreach ($this->dateFormatStorage->loadMultiple() as $format) {
-      if ($format->getPattern() == $pattern) {
+      if ($format->getPattern() == $pattern && ($this->entity->isNew() || $format->id() != $this->entity->id())) {
         form_set_error('date_format_pattern', t('This format already exists. Enter a unique format string.'));
         continue;
       }

core/modules/system/lib/Drupal/system/Tests/System/DateTimeTest.php

@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Definition of Drupal\system\Tests\System\DateTimeTest.
+ * Contains \Drupal\system\Tests\System\DateTimeTest.
  */
 
 namespace Drupal\system\Tests\System;
@@ -96,6 +96,13 @@ function testDateFormatConfiguration() {
     $this->assertText($date_format_id, 'Custom date format appears in the date format list.');
     $this->assertText(t('Delete'), 'Delete link for custom date format appears.');
 
+    // Edit the custom date format and re-save without editing the format.
+    $this->drupalGet('admin/config/regional/date-time');
+    $this->clickLink(t('Edit'));
+    $this->drupalPostForm(NULL, NULL, t('Save format'));
+    $this->assertUrl('admin/config/regional/date-time', array('absolute' => TRUE), 'Correct page redirection.');
+    $this->assertText(t('Custom date format updated.'), 'Custom date format successfully updated.');
+
     // Edit custom date format.
     $this->drupalGet('admin/config/regional/date-time');
     $this->clickLink(t('Edit'));