index.php

<?php

/*
 * Plugin Name: Ninja Forms - Datepicker Customizations
 */

add_filter( 'ninja_forms_enqueue_scripts', 'nf_datepicker_options' );
function nf_datepicker_options() {
    wp_enqueue_script( 'nf_datepicker_options', plugin_dir_url( __FILE__ ) . 'script.js', array( 'jquery' ), false, true );
}

script.js

jQuery( document ).ready( function() {
    new(Marionette.Object.extend( {

        initialize: function() {
            this.listenTo( Backbone.Radio.channel( 'pikaday' ), 'init', this.modifyDatepicker );
        },

        modifyDatepicker: function( dateObject, fieldModel ) {
            // dateObject.pikaday.setDate( '04/11/2016' );
            // dateObject.pikaday.gotoYear( '2017' );
            // dateObject.pikaday.setMinDate( new Date( 'December 21, 2016' ) );
        }
    }));
});