How to set date range in Bid Landscapes - Adwords API

I am playing with Bid Landscapes, and as the official documentation describes:

Bid landscapes are a way for you to research information about estimated performance for your ad groups and criteria.

I am testing the API in AdGroup level, and the following piece of code has been written:

public function test_bid_simulator() {      $user = new AdWordsUser();      $user->SetClientCustomerId('*******');      $dataService = $user->GetService('DataService', 'v201509');        $selector = new Selector();      $selector->fields = array('AdGroupId', 'StartDate', 'EndDate',          'Bid', 'LocalClicks', 'LocalCost', 'LocalImpressions');        // Create predicates.      $selector->predicates[] = new Predicate('CampaignId', 'IN', array('****', '****', '****', '****'));        // $selector->dateRange = new DateRange();      // $selector->dateRange->min = date('Ymd', strtotime('2016/01/28'));      // $selector->dateRange->max = date('Ymd', strtotime('2016/02/03'));        do {          // Make the getAdGroupBidLandscape request.          $page = $dataService->getAdGroupBidLandscape($selector);            // Display results.          if (isset($page->entries)) {              foreach ($page->entries as $bidLandscape) {                  printf("Found adgroup bid landscape with id '%s' for start "                          . "date '%s', end date '%s', and landscape points:\n",                          $bidLandscape->adGroupId,                          $bidLandscape->startDate,                          $bidLandscape->endDate);                  foreach ($bidLandscape->landscapePoints as $bidLandscapePoint) {                      printf("  bid: %.0f => clicks: %d, cost: %.0f, impressions: %d\n",                               $bidLandscapePoint->bid->microAmount,                               $bidLandscapePoint->clicks,                               $bidLandscapePoint->cost->microAmount,                               $bidLandscapePoint->impressions);                  }                  print "\n";              }          }          // Advance the paging index.          $selector->paging->startIndex += AdWordsConstants::RECOMMENDED_PAGE_SIZE;      } while (isset($page->entries) && count($page->entries) > 0);        if ($selector->paging->startIndex === 0) {          print "No adgroup bid landscapes were found.\n";      }  }

This code works fine, and the output is similar to:

Found adgroup bid landscape with id '****' for start date   '20160131', end date '20160206', and landscape points:    bid: 60000 => clicks: 0, cost: 0, impressions: 38    bid: 110000 => clicks: 0, cost: 0, impressions: 70    bid: 150000 => clicks: 0, cost: 0, impressions: 97    bid: 210000 => clicks: 0, cost: 0, impressions: 116    bid: 280000 => clicks: 0, cost: 0, impressions: 126    bid: 470000 => clicks: 0, cost: 0, impressions: 136    Found adgroup bid landscape with id '****' for start date '20160131', end date '20160206', and landscape points:    bid: 20000 => clicks: 0, cost: 0, impressions: 16    bid: 40000 => clicks: 0, cost: 0, impressions: 89    bid: 60000 => clicks: 0, cost: 0, impressions: 138    bid: 100000 => clicks: 0, cost: 0, impressions: 183    bid: 160000 => clicks: 0, cost: 0, impressions: 218    bid: 240000 => clicks: 0, cost: 0, impressions: 234    bid: 390000 => clicks: 0, cost: 0, impressions: 256    etc.

By default, the API always picks up the previous week range ending two days from now. For instance, if today is 2016/02/08, the API will get the range between 2016/01/31 and 2016/02/06. My problem is how to set the specific date range in the code. I have commented out this part

// $selector->dateRange = new DateRange();  // $selector->dateRange->min = date('Ymd', strtotime('2016/01/01'));  // $selector->dateRange->max = date('Ymd', strtotime('2016/01/31'));

because it doesn't work. If I uncomment this part, the code won't give me any results. Any knowledge of how to set a date range for the bid landscapes in Adwords API?

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