I think we can call this a bug. In _form.class.php, arround line 864, the code asumes that the timefmt length is 8 char, when it really depends on locale_timefmt. If you don't use seconds, then the length is 5. So what we did to solve this is:

$mytimefmt = locale_timefmt();
if( strlen($mytimefmt) == 3 )
{
	$mytime = 5;
} else {
	$mytime = 8;
}

//		$field_value = substr( $field_value, 11, 8 );
		$field_value = substr( $field_value, 11, $mytime );