I'm able to select multiple dates, then save it to database. When reading back from DB, the value attribute of the input field has all previously selected dates in it:
<input type="text" class="form-control datetimepicker-input" data-target="#datetimepicker1" value="2019-06-18,2019-06-17,2019-06-19"/>
When the page is loading you can see all 3 briefly listed, but then only 1st one remains and the other 2 disappear.
$(function() {
$('#datetimepicker1').datetimepicker({
allowMultidate: true,
multidateSeparator: ',',
format: 'YYYY-MM-DD'
});
});