Expire all membership levels at the end of the year. If the user registers after October (10 variable on line 8) then they'll expire in the next year.

pmpro-expire-at-end-of-year.php

/// EXPIRATION DATE
function my_pmpro_checkout_level($level)
{
	$expiration_year = date('Y');
	$current_month = date('n');
	
	// IF OCT, NOV, DEC, EXPIRE NEXT YEAR
	if($current_month >= 10) { $expiration_year++; }
	$expiration_date = $expiration_year . "-12-31";
	
	//needed below
	$todays_date = time();	
	
	//how many days until expiration?
	$time_left = strtotime($expiration_date) - $todays_date;
	$days_left = ceil( $time_left / (60*60*24) );
	
	//update number and period
	$level->expiration_number = $days_left;
	$level->expiration_period = "Day";

	return $level;
}
add_filter("pmpro_checkout_level", "my_pmpro_checkout_level");