Defina a caixa de texto no centro da div em bootstrap
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-alpha/css/bootstrap.css" rel="stylesheet" />
<td style="background:#B3AEB5;">
<div class="form-group text-center">
<div class="input-group" style="margin:auto;">
<input type="month" name="p2" value="test">
</div>
</div>
</td>
Ankur