Query Posts by Registration Date

I have implemented S2 member plugin and and querying posts by registration date and forward. However, I am not getting this result with the query I currently have. The first part is converting the unix time to human second part is querying the specific date forward. Would like to have the prior 30 days as well.

<?php function unix_timestamp_to_human ($timestamp = "", $format = 'Y-m-d')
{
    if (empty($timestamp) || ! is_numeric($timestamp)) $timestamp = time();
    return ($timestamp) ? date($format, $timestamp) : date($format, $timestamp);
}

$unix_time = S2MEMBER_CURRENT_USER_REGISTRATION_TIME;

  function filter_where($where = '') {
    $where .= " AND post_date > '" . date('unix_timestamp_to_human($unix_time)', strtotime('-30 days')) . "'";
    return $where;
  }
add_filter('posts_where', 'filter_where');
query_posts($query_string); ?>

Please Advise…