Cross-platform PHP-based solution:
But because smarty is a PHP plugin we can use all of PHP functions. So this is another solution I found, maybe not so elegant but it seems to work everywhere.
{assign var="dateTime" value=$variable|strtotime}
{*
or shorter:
{$dateTime=$variable|strtotime}
*}
<p>{"j/n/Y"|date:$dateTime}</p>