Prorate the inital payment & set the start date for the recurring payments to the 1st of the month

pmpro-start-of-month-payments.php

<?php
/*
Plugin Name: Prorated initial membership payment
Plugin URI: http://www.paidmembershipspro.com/wp/pmpro-customizations/
Description: Will prorate the membership payment from today until the 1st of the next month.
Version: .1
Author: Thomas Sjolshagen @ Stranger Studios <thomas@eighty20results.com>
Author URI: http://www.eighty20results.com/thomas-sjolshagen
*/

/**
 * Set the membership level to account for prorated initial payment (from today until the 1st of next month).
 *
 * @param stdClass $level - The membership level definition to update
 *
 * @return stdClass (level)
 */
function e20r_prorate_initial_payment( $level ) {

	if ( false == pmpro_hasMembershipLevel() ) {

		// user does not have level already, but we want to prorate initial payment for rest of the month
		//today
		$today = current_time("timestamp");

		//when would the first subscription payment be
		$next_payment_date = strtotime('first day of next month', current_time('timestamp'));

		//how many days left
		$days_left = ceil( ($next_payment_date - $today) / DAY_IN_SECONDS );

		// convert to percent
		$percent = $days_left / ( $next_payment_date - strtotime('first day of this month', current_time('timestamp')));

		$level->initial_payment = round( $level->initial_payment * $percent, 2);
	}

	return $level;
}
add_filter('pmpro_checkout_level', 'e20r_prorate_initial_payment', 11, 1);

/**
 * Set the startdate for the next payment for use by the payment gateway (i.e. 1st of next month)
 * @param   string          $startdate      The default start date for the subscription plan
 * @param   MemberOrder     $order          The order object
 *
 * @return string               The date to use
 */
function e20r_prorate_set_startdate( $startdate, $order ) {

	if ( false == pmpro_hasMembershipLevel() ) {

		$startdate = date("Y-m-01", strtotime(date("Y-m-01", current_time('timestamp')) . " + " . $order->BillingFrequency . " " . $order->BillingPeriod)) . "T0:0:0";
	}

	return $startdate;
}
add_filter('pmpro_profile_start_date', 'e20r_prorate_set_startdate', 11, 2 );