How to center content with dynamic CSS dimensions?

Question

I am using Bootstrap 4, which is based on flexbox(dynamic properties/dimensions). I created a Datapicker, with an input and icon on the right side.

<div class="col-md-3">
   <div class="input-group date" data-provide="datepicker" style="flex: 0 0 auto;">
      <input type="text" class="form-control" class="datepicker">
      <div class="input-group-addon" style="justify-content: center;align-items: center;flex: 0 0 25px;">
         <i class="fa fa-md fa-calendar" aria-hidden="true"></i>
      </div>
   </div>
</div>

When I try to center the icon, with

style="justify-content: center;align-items: center;flex: 0 0 25px;"

the image looks like this

console.log($date = "data");
$('.datepicker').datepicker({
		format: 'dd/mm/yyyy'
}).on('changeDate', function(e) {
$date = $('.datepicker').datepicker('getDate');
		console.log($date);
});

<!-- Bootstrap CND -->
	<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
  <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
	<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
	<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
	<!-- Datapicker css -->
	<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.8.0/css/bootstrap-datepicker.css">
  <script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.8.0/js/bootstrap-datepicker.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.8.0/locales/bootstrap-datepicker.pt.min.js"></script>
  <link href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">

<div class="container-fluid">
  <div class="row">
    <div class="col-md-3">
	    <div class="input-group date" data-provide="datepicker" id="sandbox-container" style="flex: 0 0 auto;">
			<input type="text" class="form-control" class="datepicker" data-date-format="dd/mm/yyyy" data-date-autoclose="true" data-date-today-highlight="true">
			<div class="input-group-addon" style="justify-content: center;align-items: center;flex: 0 0 25px;">
				<i class="fa fa-calendar" aria-hidden="true" style="align-self: stretch"></i>
			</div>
		</div>
	  </div>
  </div>
</div>

[update] 02-03-2019

Replace

<div class="input-group-addon" style="justify-content: center;align-items: center;flex: 0 0 25px;">

for

<div class="input-group-addon d-flex  align-items-center justify-content-center" style="flex: 0 0 25px">

Answer 1

It is not working because these classes of flex you have to use on the father you have display:flex. Also, there is no need to use the styles in hand, as Bootstrap itself has classes of flex to make this alignment correctly using the classes d-flex and align-items-center as you can see here https://getbootstrap.com/docs/4.0/utilities/flex/

<link rel="stylesheet" type="text/css" media="screen" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" />
<link rel="stylesheet" type="text/css" media="screen" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" />

<div class="col-md-3">
    <div class="input-group date" data-provide="datepicker" style="flex: 0 0 auto;">
        <input type="text" class="form-control" class="datepicker">
        <div class="input-group-addon d-flex  align-items-center" style="">
            <i class="fa fa-md fa-calendar ml-2" aria-hidden="true"></i>
        </div>
    </div>
</div>