Could not use my own view helper in Volt

Hi, I write my own view helper as below:

<?php
namespace Tartan;

class Tag extends \Phalcon\Tag
{
    static public function gtoj($date)
    {
        [...]
        return $response;
    }
}

then I added my own tag service in DI in my application bootstrap:

$di->set('tag', function() {
    return new \Tartan\Tag();
});

NOW, I can use my helper like below and it works perfectly

<?php echo $this->tag->gtoj('2014-03-03'); ?>

but I can`t use it in Volt like

{{ gtoj("2014-03-03") }}

and I got following error:

Phalcon\Mvc\View\Exception: Undefined function 'gtoj' in /home/aboozar/www/2013/blue/app/modules/Panel/views/merchant/index.phtml on line 17

other tag helpers like {{ link_to("products/search", "Search") }} works in the same file too.

any Idea?