Set billing start date (subscription delay) to expiration date when member is renewing for recurring level

pmpro-recurring-level-sub-delay-example.php

<?php
/**
 * Move the members subscription start date based on their remaining days of current level that has an expiration date.
 * Note: Do not set initial payment to $0, let members pay ahead for their next billing period before starting subscriptions.
 * Useful for any migration where we cannot migrate the subscription and need to checkout again.
 */
function my_pmpro_renewal_defer_startdate( $checkout_level ) {
    // Only run on the frontend during checkout
    if ( ! function_exists( 'pmpro_getMembershipLevelForUser' ) ) {
        return $checkout_level;
    }

    $user_id = get_current_user_id();
    if ( ! $user_id ) {
        return $checkout_level;
    }

	// Only for recurring levels.
	if ( ! pmpro_isLevelRecurring( $checkout_level ) ) {
		return $checkout_level;
	}

    // Get the member's current level
    $current_level = pmpro_getMembershipLevelForUser( $user_id );

    // Only proceed if they already hold this exact level (renewal scenario)
    if ( empty( $current_level ) || (int) $current_level->id !== (int) $checkout_level->id ) {
        return $checkout_level;
    }

    // Only defer if there's a future expiration date
    if ( empty( $current_level->enddate ) || $current_level->enddate === '0000-00-00 00:00:00' ) {
        return $checkout_level;
    }

	// Seconds remaining on the current level from today until the enddate.
	$days_remaining = $current_level->enddate - time();

	// If there are any days remaining, they still pay the full initial amount BUT they also get their subscription first payment date to be delayed for remaining days.
	if ( $days_remaining > 0 ) {
		// The original first-billing date is now + cycle_number cycle_period (e.g. +1 Year).
		$original_startdate = strtotime( '+' . (int) $checkout_level->cycle_number . ' ' . $checkout_level->cycle_period );
		$new_startdate = $original_startdate + $days_remaining;

		$checkout_level->startdate = date( 'Y-m-d', $new_startdate );
		// Adjust the text at checkout to show a small hint that billing will only start on a specific date.
		add_filter( 'pmpro_level_cost_text', 'my_pmpro_show_checkout_level_startdate_in_cost_text', 10, 2 );
	}
    
    return $checkout_level;
}
add_action( 'pmpro_checkout_level', 'my_pmpro_renewal_defer_startdate' );

// Show the checkout level startdate in the cost text so customer's know.
function my_pmpro_show_checkout_level_startdate_in_cost_text( $cost, $level ) {
	if ( ! empty( $level->startdate ) ) {
		$start_date = date_i18n( get_option( 'date_format' ), strtotime( $level->startdate ) );
		$cost      .= sprintf( ' <small>(%s %s)</small>', esc_html__( 'Billing starts on', 'paid-memberships-pro' ), esc_html( $start_date ) );
	}
	return $cost;
}