<input type="date" name="data-inicial">
<input type="date" name="data-final">

<input type='date' id='d1' name='d1' min="">
<input type='date' id='d2' name='d2' min="">

<script>
  $(document).ready(function() {
      $("#d1").change(function () {
        var split = $("#d1").val().split('-');
        var fim = new Date(parseInt(split[0]), parseInt(split[1]), parseInt(split[2]) + 29);
        $("#d2").attr({
           "min" : [fim.getFullYear(),
                (fim.getMonth() > 9 ? '' : '0') + fim.getMonth(),
                (fim.getDate() > 9 ? '' : '0') + fim.getDate()]
                .join('-')
        });
      });
  });
</script>