Overloading the selectFields method in your model

controller.php

<?php

$loader = require __DIR__."/../vendor/.composer/autoload.php";
$loader->add('YourDb', "/tmp"); // Set yours


use YourDb\Blog\Post;
use YourDb\Blog\PostMap;

$database = new Pomm\Connection\Database(array('dsn' => 'pgsql://user:password@host:port/db_name'));

$posts = $database
    ->getConnection()
    ->getMapFor('YourDb\Blog\Post')
    ->findAll('post')
    ;

foreach($posts as $post)
{
    printf("Title: %s, posted %s days ago.\n", $post->getTitle(), $post->getCreatedSince());
}

PostMap.php

<?php

namespace YourDb\Blog;

use YourDb\Blog\Base\PostMap as BasePostMap;
use Pomm\Exception\Exception;
use Pomm\Query\Where;

class PostMap extends BasePostMap
{
    public function getSelectFields($alias = null)
    {
        $fields = parent::getSelectFields($alias);
        $db_alias = is_null($alias) ? '' : sprintf('%s.', $alias);
        $fields['created_since'] = sprintf("date_part('d', now() - %screated_at)", $db_alias)

        return $fields;
    }
}