There isn’t any way to do this in ACF. What you’re going to need to do is to create a new custom widget. You can start by copying the existing calendar widget /wp-includes/widgets/class-wp-widget-calendar.php
WP Widget API doc: https://codex.wordpress.org/Widgets_API
Widget turorial: http://www.wpbeginner.com/wp-tutorials/how-to-create-a-custom-wordpress-widget/