Sort posts by date that has been taken from a bootstrap calendar

Hello!

I have create a small backend system to manage a web page. When i post something i add the date by using a bootstrap calendar plugin to a text input. What i wanted is to sort all posts in a descending order both in backend and frontend. It seems that this doesn't happen. The date form i'm using is the day/month/year.

Here is the table for the posts :

public function up()
    {
    Schema::create('posts', function (Blueprint $table) {
        $table->increments('id');
        $table->string('postDate');
        $table->string('priority');
        $table->string('postThumb');
        $table->string('elPostTitle');
        $table->text('elPostContent');
        $table->string('enPostTitle');
        $table->text('enPostContent');
        $table->string('uri');
        $table->rememberToken();
        $table->timestamps();
    });
}

Also here is the way i retrieve them from the Controller

public function getPosts(){
    $response = array(
        'status' => 0,
        'message' => array(),
        'posts' => array()
        );

    // Get all posts Data
    $posts = new Post;
    array_push($response['posts'], $posts::orderBy('postDate', 'DESC')->get());
    return json_encode($response);
}

And at last. Here is an example of how it returns the posts order by desc date.

24/06/2005
22/05/2006
19/08/2011
18/07/2000
17/03/2008
15/09/2015
14/07/2010
14/07/1993
13/06/2007
13/01/2016
11/07/2007
10/08/2010
10/05/2010
10/01/2012
08/07/1998
07/06/2006

Is there any quick/easy way to fix this ?

Any idea/hint is welcomed!

Thank you.