On some installations where PHP was compiled without cal support, this mod will fail because the cal_days_in_month function doesn't exist. You can just define it locally if this happens to you.

Under the inclusion of application_top, add

Code:
if (!function_exists('cal_days_in_month')) {
  function cal_days_in_month($cal, $month, $year) {
    return date('t', mktime(0, 0, 0, $month, 1, $year));
  }
}