Prorate initial payment based on subscription delay.

my_pmpro_checkout_level.php

<?php

/**
 * Prorate initial payment based on subscription delay.
 *
 * Requires Subscription Delay and Proration Add Ons.
 */
function my_pmpro_checkout_level( $level ) {

	// Only continue if Subscription Delays and Proration Add Ons are activated.
	if ( ! function_exists( 'pmprosd_convert_date' ) || ! function_exists( 'pmprorate_trim_timestamp' ) ) {
		return $level;
	}

	// Only continue for new members. (no "success" orders for this level)
	global $current_user;

	if ( ! empty( $current_user ) ) {
		$order = new MemberOrder();
		$order->getLastMemberOrder( $current_user->ID );

		if ( ! empty( $order ) && $order->membership_id == $level->id ) {
			return $level;
		}
	}

	// Get subscription start date for level.
	$subscription_delay = get_option( 'pmpro_subscription_delay_' . $level->id );

	if ( empty( $subscription_delay ) ) {
		return $level;
	}

	if ( ! is_numeric( $subscription_delay ) ) {
		$start_date = pmprosd_convert_date( $subscription_delay );
	} else {
		$start_date = date( 'Y-m-d', strtotime( '+ ' . intval( $subscription_delay ) . ' Days', current_time( 'timestamp' ) ) ) . 'T0:0:0';
	}

	// Prorate initial payment.
	$next_payment_date = pmprorate_trim_timestamp( strtotime( $start_date, current_time( 'timestamp' ) ) );
	$payment_date      = pmprorate_trim_timestamp( strtotime( "-{$level->cycle_number} {$level->cycle_period}", $next_payment_date ) );
	$today             = pmprorate_trim_timestamp( current_time( 'timestamp' ) );

	$days_in_period = ceil( ( $next_payment_date - $payment_date ) / 3600 / 24 );

	// if no days in period (next payment should have happened already) return level with no change to avoid divide by 0
	if ( $days_in_period <= 0 ) {
		return $level;
	}

	$days_passed = ceil( ( $today - $payment_date ) / 3600 / 24 );
	$per_passed  = $days_passed / $days_in_period;        // as a % (decimal)
	$per_left    = 1 - $per_passed;

	$credit = $level->initial_payment * $per_passed;

	$level->initial_payment = round( $level->initial_payment - $credit, 2 );

	// just in case we have a negative payment
	if ( $level->initial_payment < 0 ) {
		$level->initial_payment = 0;
	}

	// Make sure we don't run other proration code if we get to this point.
	remove_filter( 'pmpro_checkout_level', 'pmprorate_pmpro_checkout_level', 10, 1 );

	return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level', 5 );