Formata de maneira customizada intervalos de data.

DateFormatter.php

<?php

ini_set('date.timezone','America/Sao_Paulo');

class DateFormatter extends DateTime
{
    private $parts = array(
        array(31104000, 'ano', 'anos'),
        array(2592000, 'mês', 'meses'),
        array(86400, 'dia', 'dias'),
        array(3600, 'hora', 'horas'),
        array(60, 'minuto', 'minutos'),
        array(1, 'segundo', 'segundos'),
    );

    public function formattedInterval(DateTime $relativeTo = NULL)
    {
        if (is_null($relativeTo)) {
            $relativeTo = new DateTime();
        }
        $diff       = parent::format('U') - $relativeTo->format('U');
        $past       = FALSE;
        if ($diff < 0) {
            $past   = TRUE;
            $diff  *= -1;
        }
        foreach ($this->parts as $subparts) {
            $n = floor($diff / $subparts[0]);
            if ($n) {
                $output = '%d %s';
                $part   = $subparts[$n > 1 ? 2 : 1];
                if ($past) {
                    $part  .= ' atrás';
                }
                return sprintf($output, $n, $part);
            }
        }
        return 'agora';
    }
}

$d = new DateFormatter('10-May-2011');

var_dump($d->formattedInterval());