This custom snippet adjusts the expiration dates for Paid Memberships Pro memberships based on a specific cutoff date.

pmpro-expiration-date-cutoff-adjustment.php

<?php
/**
 * Adjust expiration date based on a defined cutoff date for Paid Memberships Pro.
 *
 * If a user signs up or renews on or after the defined cutoff date,
 * their membership expiration date is extended by a specified period
 * (e.g., December 31 of the next year).
 * If they sign up or renew before the cutoff, the expiration date
 * is set to a different period (e.g., December 31 of the current year).
 *
 * Example: Using October 1 as the cutoff date.
 *
 * To use this snippet, add it via a custom plugin or
 * the Code Snippets plugin.
 */

function my_pmpro_adjust_expiration_date_based_on_cutoff( $set_date ) {
    // Define the cutoff date (e.g., October 1).
    $current_year = date( 'Y' );
    $cutoff_date = strtotime( $current_year . '-10-01' ); // Replace '10-01' with your desired cutoff date (e.g., MM-DD).
    $current_date = time(); // Current timestamp.

    // Get the current expiration date.
    $expiration_date = strtotime( $set_date );

    // Adjust expiration date based on the cutoff date.
    if ( $current_date >= $cutoff_date ) {
        // Set expiration to December 31 of the next year.
        $set_date = date( 'Y-12-31', strtotime( '+1 year', $expiration_date ) );
    } else {
        // Set expiration to December 31 of the current year.
        $set_date = date( 'Y-12-31', $expiration_date );
    }

    return $set_date;
}
add_filter( 'pmprosed_expiration_date', 'my_pmpro_adjust_expiration_date_based_on_cutoff' );