Q:

How to create dynamic datetimepicker value?

My HTML code like this:

$(function () {                
    $('#datetimepicker').datetimepicker({
        minDate: moment().startOf('minute').add(300, 'm'),
    });
    
    $("#btnSubmit").click(function() {
        value = document.getElementById('datetimepicker').value;
        console.log(value)
    });
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.13.0/moment.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datetimepicker/4.17.37/js/bootstrap-datetimepicker.min.js"></script>

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet"/>
<link href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datetimepicker/4.17.37/css/bootstrap-datetimepicker.min.css" rel="stylesheet"/>


<div class="container">
    <div class="row">
        <div class='col-sm-6'>
            <div class="form-group">
                <input type='text' class="form-control" id='datetimepicker' />
            </div>
        </div>
    </div>
</div>
<button id="btnSubmit">
    Submit
</button>

Demo like this  : https://jsfiddle.net/oscar11/8jpmkcr5/83/
If I click submit button, I successfully get the datetime value 300 minutes from now
But if before submitting, I wait 10 minutes, the result does not match my expectations
For example now at 6 o'clock. Before submit I wait 10 minutes. After submit, the time value I get is 11.10. So I want to like it
How can I do that?
How do I make the automatic datetime value increase?
Update :
Whether the time value in textfield can be automatically increased based on current time? 

A:

Not sure why would you need it, but you can use something like:
$("#btnSubmit").click(function() {
    var targetDate = new Date(new Date().getTime() + (300 * 60 * 1000));
    $('#datetimepicker').data("DateTimePicker").date(targetDate);

   value = document.getElementById('datetimepicker').value;
   console.log(targetDate, value);
});

On click, it will change the datetimepicker to the current date + 300 minutes.
I've updated the example: https://jsfiddle.net/8jpmkcr5/86/
EDIT:
Based on your latest comment, i believe you want the use to be able to pick the date (day/month/year) but the time (hour/minutes) keep changing as time passes to current time + 3 hours. If thats the case you can use something like:
$(function () {                
  $('#datetimepicker').datetimepicker({
        minDate: moment().startOf('minute').add(180, 'm'),
    });
});

setInterval(function(){
    var pickedDate = $('#datetimepicker').data('DateTimePicker').date();
    var currentDate = moment();
    pickedDate = pickedDate.set({
            'hour' : currentDate.get('hour') + 3,
            'minute'  : currentDate.get('minute'), 
            'second' : currentDate.get('second')
         });
    $('#datetimepicker').data("DateTimePicker").date(pickedDate);
 },1000);

You can test here: https://jsfiddle.net/8jpmkcr5/98/

