Custom proration billing amount plus initial amount for a level with a fixed renewal date (like “Y1-10-01”). Requires Subscription Delays Add On.

pmprosd_prorate_fixed_annual-delay.php

<?php
/**
 * PMPro Customization: Prorate Initial Payment Based on Fixed Renewal Date. requires Subscription Delays Add On
 *
 * This recipe adjusts the initial payment for a membership level to reflect a prorated charge
 * from the join date until a fixed annual renewal date (e.g., October 1). 
 * The membership level should have its Subscription Delay set to: `Y1-10-01`
 */
function my_pmprosd_prorate_delay( $level ) {
	// Only apply to specific level ID(s) — update as needed.
	if ( $level->id != 1 ) {
		return $level;
	}

	// Define fixed annual renewal date (MM-DD format).
	$subscription_day = "10-01";
	$now = new DateTime();

	// Skip proration if today is already the renewal date.
	if ( $now->format( 'm-d' ) === $subscription_day ) {
		return $level;
	}

	// Determine correct target year for the renewal.
	$start_year = $now->format( 'm-d' ) >= $subscription_day ? $now->format( 'Y' ) + 1 : $now->format( 'Y' );
	$subscription_start = new DateTime( $start_year . '-' . $subscription_day );

	// Calculate how much of the year remains until the renewal date.
	$interval_seconds   = $subscription_start->getTimestamp() - time();
	$proration_fraction = $interval_seconds / ( 365 * 24 * 60 * 60 );

	// Avoid overcharging or invalid values.
	if ( $proration_fraction <= 0 || $proration_fraction > 1 ) {
		return $level;
	}

	// Dynamically pull billing and signup fee from level settings.
	$billing_amount = isset( $level->billing_amount ) ? (float) $level->billing_amount : 0;
	$signup_fee     = isset( $level->initial_payment ) ? (float) $level->initial_payment : 0;

	// Determine base fee portion (signup fee minus one billing cycle, if applicable).
	$base_fee = max( 0, $signup_fee - $billing_amount );

	// Final calculated prorated initial charge.
	$level->initial_payment = round( ( $billing_amount * $proration_fraction ) + $base_fee, 2 );

	return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmprosd_prorate_delay' );