Looking to rewrite some PHP code to include in a .tpl file

Is it possible to adjust the following code to work in a .tpl (smarty) file?

  function getinvoicepayuntildate ($nextduedate, $billingcycle, $fulldate = '')
  {
    $year = substr ($nextduedate, 0, 4);
    $month = substr ($nextduedate, 5, 2);
    $day = substr ($nextduedate, 8, 2);
    $daysadjust = 0;
    if (!$fulldate)
    {
      $daysadjust = 1;
    }

    if ($billingcycle == 'Monthly')
    {
      $new_time = mktime (0, 0, 0, $month + 1, $day - $daysadjust, $year);
    }
    else
    {
      if ($billingcycle == 'Quarterly')
      {
        $new_time = mktime (0, 0, 0, $month + 3, $day - $daysadjust, $year);
      }
      else
      {
        if ($billingcycle == 'Semi-Annually')
        {
          $new_time = mktime (0, 0, 0, $month + 6, $day - $daysadjust, $year);
        }
        else
        {
          if ($billingcycle == 'Annually')
          {
            $new_time = mktime (0, 0, 0, $month, $day - $daysadjust, $year + 1);
          }
          else
          {
            if ($billingcycle == 'Biennially')
            {
              $new_time = mktime (0, 0, 0, $month, $day - $daysadjust, $year + 2);
            }
          }
        }
      }
    }

    if ($billingcycle != 'One Time')
    {
      $invoicepayuntildate = date ('Y-m-d', $new_time);
    }

    return $invoicepayuntildate;
  }

Code (markup):

Thanks for any help.