Why my dropdown list can't be position in center?

I want to make the position of my dropdown list to be center. But when I made

align="center"

the posistion isn't in the center

Here is my codes:

<?php

/* @var $this yii\web\View */



$this->title = 'Beranda - Pascasarjana Dalam Angka';
?>
<div class="site-index">

    <div class="jumbotron">
        <h1>Selamat Datang!</h1>

        <p class="lead-lg-5">Aplikasi ini ditujukan untuk menampilkan <br>data-data dan statistik mahasiswa program Pascasarjana Universitas</p>


    <div align="center" class="body-content">


    <div align="center" class="dropdown">
        <button align="center" class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Pilih Tahun Periode
            <span class="caret"></span>
        </button>
                <ul align="center" class="dropdown-menu">
                    <li><a href="#">Periode 2001-2005</a></li>
                    <li><a href="#">Periode 2006-2010</a></li>
                    <li><a href="#">Periode 2011-2015</a></li>
                </ul>
    </div>



    </div>


    </div>

</div>

And here is the result:

What may I do so that the list could be in the center? Thanks in advance