Most of bootstrap datepickers as I write this answer are rather buggy when included in Bootstrap 4.
In my view the least code adding solution is a jQuery plugin.
I used this one https://plugins.jquery.com/datetimepicker/ - you can see its usage here: https://xdsoft.net/jqplugins/datetimepicker/
It sure is not as smooth as the whole BS interface, but it only requires its css and js files along with jQuery which is already included in bootstrap.
