[REF] Fix second output in customFormat function and extend unit test

CRM/Utils/Date.php

@@ -369,7 +369,7 @@ class CRM_Utils_Date {
 
         $hour24 = (int) substr($dateString, 11, 2);
         $minute = (int) substr($dateString, 14, 2);
-        $second = (int) substr($dateString, 16, 2);
+        $second = (int) substr($dateString, 17, 2);
       }
       else {
         $year = (int) substr($dateString, 0, 4);

tests/phpunit/CRM/Utils/DateTest.php

@@ -270,6 +270,7 @@ class CRM_Utils_DateTest extends CiviUnitTestCase {
     $this->assertEquals(CRM_Utils_Date::customFormat($dateTime, "%p"), "pm");
     $this->assertEquals(CRM_Utils_Date::customFormat($dateTime, "%P"), "PM");
     $this->assertEquals(CRM_Utils_Date::customFormat($dateTime, "%Y"), "2018");
+    $this->assertEquals(CRM_Utils_Date::customFormat($dateTime, "%s"), "44");
   }
 
   /**