Prorate the initial payment. Useful for subscriptions that occur on the first of every month.

pmpro-prorate-initial-payment.php

/**
 * Prorate the initial payment at PMPro checkout based on the day of the month.
 * You should use the Subscription Delays add on to set your subscription
 * to delay until Y1-M1-01.
 */
function my_pmpro_checkout_level( $level ) {	
	$current_day = date( 'j' );	

	// Ignore if it's the first of the month.
	if ( $current_day == 1 ) {
		return $level;
	}

	$days_passed = $current_day - 1;
	$days_in_month = date( 't' );

	$level->initial_payment = pmpro_round_price( $level->initial_payment * ( ( $days_in_month - $days_passed ) / $days_in_month ) );
	
	return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level' );