This works for me after a long hours of search , This solution is for Bootstrap datetimepicker version 4. when you have to set the default date in input field.
HTML
"input type='text' class="form-control" id='datetimepicker5' placeholder="Select to date" value='<?php echo $myDate?>'// $myDate is having value '2016-02-23' "
Note- If You are coding in php then you can set the value of the input datefield using php, but datetimepicker sets it back to current date when you apply datetimepicker() function to it. So in order to prevent it, use the given JS code
JAVASCRIPT
<script>
$('#datetimepicker5').datetimepicker({
useCurrent: false //this is important as the functions sets the default date value to the current value
,format: 'YYYY-MM-DD'
});
</script>