/**
  * get string representation of first and last days of the week defined by date/week number
  * 
  * @param Tinebase_DateTime $_date
  * @param integer $_weekNumber optional
  * @return array
  */
 protected function _getFirstAndLastDayOfWeek(Tinebase_DateTime $_date, $_weekNumber = NULL)
 {
     $firstDayOfWeek = $this->_getFirstDayOfWeek();
     if ($_weekNumber !== NULL) {
         $_date->setWeek($_weekNumber);
     }
     $dayOfWeek = $_date->get('w');
     // in some locales sunday is last day of the week -> we need to init dayOfWeek with 7
     $dayOfWeek = $firstDayOfWeek == 1 && $dayOfWeek == 0 ? 7 : $dayOfWeek;
     $_date->sub($dayOfWeek - $firstDayOfWeek, Tinebase_DateTime::MODIFIER_DAY);
     $firstDay = $_date->toString($this->_dateFormat);
     $_date->add(6, Tinebase_DateTime::MODIFIER_DAY);
     $lastDay = $_date->toString($this->_dateFormat);
     $result = array($firstDay, $lastDay);
     return $result;
 }