How to send data with ajax datetimepicker bootstrap?

Question

Bootstrap

I have a calendar on the page

<div class="form-group">
                    <!-- элемент input с id = datetimepicker1 -->
                    <div class="input-group" id="datetimepicker1">
                        <input type="text" class="form-control" />
                        <span class="input-group-addon">
                    <span class="glyphicon glyphicon-calendar"></span>
                        </span>
                    </div>
                </div>

I need to send date to php file. And when choosing another date, send another date to the datatimepicker.

I climbed and found that the datetimepicker has an event that fires when the date changes -dp.change

I'm a noob in js, but I did it like this

$(function () {

    $('#datetimepicker1').datetimepicker(
        {format: 'DD.MM.YYYY', locale: 'ru'}


  );

    $('#datetimepicker1').on("dp.change", function ()
    {
        var a = $("#datetimepicker1").data("datetimepicker").getDate();
        $.ajax(
            {
                url:'index.php?r=category/viewdoc',
                data: a,
                type: 'GET',
                success: function (res) {
                    console.log(res);
                },
                error:function () {
                    alert('Error');
                }
            }
        );
    });
});

But it gives an error. Even if I write the text to the variable a. How to correctly issue ajax when changing the date?