Add possibility to show x weeks in the future / past

You’re already doing it partially in the code (P, C and N), but I adapted it slightly like:

    $wn_sign = substr($parameters['weeknummer'], 0, 1);
    if(
      isset($parameters) &&
      array_key_exists('weeknummer', $parameters) &&
      ($parameters['weeknummer'] === 'P' || $parameters['weeknummer'] === 'C' || $parameters['weeknummer'] === 'N' || $wn_sign === '-' || $wn_sign === '+')
    ) {
      $adjustment = str_replace(array('C', 'P', 'N'), array('+0', '-7', '+7'), $parameters['weeknummer']);
      $wn = ltrim(date('W', strtotime($adjustment.' days', $timestamp)), '0');
      $extra = str_replace(array('weeknummer='.$parameters['weeknummer']),
                           array('weeknummer='.$wn),
                           $extra);
      $parameters['weeknummer'] = $wn;
    }

So I can fill in ‘+14’ in a tag to get 2 weeks in advance. It would be better to make it ‘+2’, been lazy so far.