i copied your code, named it function.get_duedate.php in si/templates/invoices/default/plugins folder.

<?php<br />/**
* Smarty plugin
* @package Smarty
* @subpackage plugins
* @author Thomas Hilburger, www.tnt-designstudio.de
*/

/**
* Due date plugin
*
* @param mixed $params Array with 'invoiceId' and 'length'. Length will be defaulted to 6, if not set.
* @param Object $smarty
* @example {invoiceNumber invoiceId=$invoice.id length=8}
*/
function smarty_function_get_duedate($params, &$smarty)
{

if(empty($params['days']))
{
$params['days'] = 14;
}

$timeInvoice = strtotime($params['invoiceDate']."+".$params['days']." days");

echo date("d.m.Y", $timeInvoice);
}
?>

then i add a line
{get_duedate invoiceDate=$invoice.date days=14} to si/templates/invoices/defaults/template.tpl

give me a blank page. what did i do wrong?

i copy your code to si/templates/invoices/default/plugins folder
and name it function.due_date.php
<?php<br />
//function smarty_function_due_date($params, &$smarty)
/**
* Function: due_date
*
* Calculates the due date by adding the amount of days to the unix timestamp.
*
* Arguments:
*
* date - the default date output - in this format: eg: 25 May 2011
* days - amount of days to due date - eg: 14
*
* How to add to template:
*
* Paste this where you wish the due date to appear:
*
* {due_date date=$invoice.date days=14}
*
**/

function smarty_function_due_date($params, &$smarty)
{
if ($params['date'] != null AND $params['days'] != null) {

$today = htmlsafe($params[date]);
$days_to_add = htmlsafe($params[days]);

$timestamp = strtotime($today);
$due_date_stamp = strtotime($days_to_add.' day', $timestamp);
echo date('d M Y', $due_date_stamp);
}
}
?>

then I put this line in si/templates/invoice/default/template.tpl

result : blank page, what did i do wrong?