Edit in JSFiddle

var input;
$('#myModal').on('shown.bs.modal', function (e) {
    if (!input) {
        input1 = new wijmo.input.InputDate('#InputDate');
        input2 = new wijmo.input.InputDateTime('#InputDateTime');
    }
});

<button type="button" data-toggle="modal" data-target="#myModal" class="btn btn-primary">ダイアログの表示</button>
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
    <div class="modal-dialog" role="document">
        <div class="modal-content">
            <div class="modal-body">
                <input id="InputDate" type="text" />
                <input id="InputDateTime" type="text" />
            </div>
        </div>
    </div>
</div>