diff --git a/core/modules/system/lib/Drupal/system/Controller/TimezoneController.php b/core/modules/system/lib/Drupal/system/Controller/TimezoneController.php
index e58a28c..e1c16e2 100644
--- a/core/modules/system/lib/Drupal/system/Controller/TimezoneController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/TimezoneController.php
@@ -17,6 +17,19 @@ class TimezoneController {
   /**
    * Retrieve a JSON object containing a time zone name given a timezone abbreviation.
    *
+   * @param string $abbreviation
+   *   Time zone abbreviation.
+   * @param $offset
+   *   Offset from GMT in seconds. Defaults to -1 which means that first found
+   *   time zone corresponding to abbr is returned.
+   *   Otherwise exact offset is searched and only if not found then the first
+   *   time zone with any offset is returned.
+   * @param null|bool $is_daylight_saving_time
+   *   Daylight saving time indicator. If abbr does not
+   *   exist then the time zone is searched solely by
+   *   offset and isdst.
+   *
+   * @return JsonResponse
    */
   public function getTimezone($abbreviation = '', $offset = -1, $is_daylight_saving_time = NULL) {
     // An abbreviation of "0" passed in the callback arguments should be