I think I may have solved the issue.
Commented out the link set by Datepicker : <script src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
Replaced it with <script src="js/jquery-3.3.1.min.js"></script>
Commented out <script src="js/jquery-3.3.1.min.js"></script> that Bootstrap placed at page bottom.
So far it's working.