diff --git a/core/modules/system/lib/Drupal/system/DateFormatInterface.php b/core/modules/system/lib/Drupal/system/DateFormatInterface.php
index 93b2225..6801494 100644
--- a/core/modules/system/lib/Drupal/system/DateFormatInterface.php
+++ b/core/modules/system/lib/Drupal/system/DateFormatInterface.php
@@ -16,46 +16,72 @@
 interface DateFormatInterface extends ConfigEntityInterface {
 
   /**
+   * Gets the date pattern string for this format.
+   *
    * @param string $type
+   *   The date pattern type to set.
    *
    * @return string
+   *   The pattern string as expected by date().
    */
   public function getPattern($type = DrupalDateTime::PHP);
 
   /**
+   * Sets the date pattern for this format.
+   *
    * @param string $pattern
+   *   The date pattern to use for this format.
    * @param string $type
+   *   The date pattern type to set.
    *
    * @return self
+   *   Returns the date format.
    */
   public function setPattern($pattern, $type = DrupalDateTime::PHP);
 
   /**
+   * Adds a locale for this date format.
+   *
    * @param string $locale
+   *   The locale to add for this format.
    *
    * @return self
+   *   Returns the date format.
    */
   public function addLocale($locale);
 
   /**
+   * Sets the locales for this date format. This overwrites existing locales.
+   *
    * @param array $locales
+   *   The array of locales to set for this format.
    *
    * @return self
+   *   Returns the date format.
    */
   public function setLocales(array $locales);
 
   /**
+   * Returns an array of the locales for this date format.
+   *
    * @return array
+   *   An array of locale names.
    */
   public function getLocales();
 
   /**
+   * Determines if this data format has any locales.
+   *
    * @return bool
+   *   TRUE if the date format has locales, FALSE otherwise.
    */
   public function hasLocales();
 
   /**
+   * Determines if this date format is locked.
+   *
    * @return bool
+   *   TRUE if the date format is locked, FALSE otherwise.
    */
   public function isLocked();