Laravel Query Builder - Query on Date Not Working

I have a query like this in Laravel Query Builder-

    $baseQuery  =   DB::table(  'project');
    //$startDate="2015-09-02" - This format
    //$endtDate="2014-08-02" - This format
    if(!empty($startDate))
        $baseQuery  =   $baseQuery->where('project.completion_date', '>', $startDate);
    if(!empty($endData))
        $baseQuery  =   $baseQuery->where('project.completion_date', '<', $endData);

But it is not working perfectly, can anyone help me please by telling what I am doing wrong?

via Chebli Mohamed