Set Default Due Date to 1st of Next Month

functions.php

<?php // don't add this line since it's already in your functions.php file

/**
 * Edited invoices
 */
function _si_information_meta_box_args( $args ) {
	if ( 'auto-draft' == $args['post']->post_status ) { // only adjust drafts
		$args['due_date'] = strtotime('first day of next month');
	}
	return $args;
}
add_filter( 'load_view_args_admin/meta-boxes/invoices/information.php', '_si_information_meta_box_args' );
add_filter( 'load_view_args_admin/meta-boxes/estimates/information.php', '_si_information_meta_box_args' );


/**
 * Generated invoices
 */
function _default_due_in_days() {
  $first_day_of_next_month = strtotime('first day of next month');
  $days_till_next_month = ($first_day_of_next_month - time()) / (DAY_IN_SECONDS);
	return (int) $days_till_next_month;
}
add_filter( 'si_default_due_in_days', '_default_due_in_days' );

This snippet is a customization for Sprout Invoices. A customizable solution that provides a way for you to get paid via your WordPress site. For more information please don't hesitate to reach out.