Dynamically adjust WooCommerce Subscriptions first synced payment date

gistfile1.php

/**
 * I have a WooCommerce Subscription product set up to bill every 2nd month
 * The product is also set up to synchronize payments to the 25th of each month
 * and the products are shipped on the 1st of every 2nd month - the shipping
 * schedule is fixed, i.e. shipments only go out on even numbered months
 * 
 * How can I prevent my customers from being billed 5 weeks before their first box
 * actually goes out?
 */
add_filter( 'woocommerce_subscriptions_synced_first_payment_date', 'wooninja_filter_first_payment_date', 10, 2 );

function wooninja_filter_first_payment_date( $first_payment_date, $product ) {

	$day = (int) date( 'j', $first_payment_date );
	$month = (int) date( 'n', $first_payment_date );
	//Edit the product id (666) below as necessary
	//the day condition is to prevent trying to increment
	//a month if that might result in an invalid date string
	if ( 666 == $product->id && 0 == $month % 2 && $day <= 28 ) {
		$first_payment_date = strtotime( $month + 1 . '/' . date( 'j/Y H:i:s', $first_payment_date ) );
	}
	return $first_payment_date;
}