Ticket price increase countdown not outputting positive integer

Hello, I’m creating a ticket countdown, I’ve got it to work (nearly!) I’m just struggling to get the $diff variable to be a positive or negative integer, and wondered if you could help? Currently my code is OK for future dates, but once the date has passed – the $diff variable is still a positive integer, so the else {} block isn’t firing.

any ideas? (Thanks in advance!)

<?php
			$today = new DateTime('now');
			$date = new DateTime(get_field( 'tier_increase_date', 'option' ));

			$diff = $date->diff($today)->format("%a");

			if ( $diff >= 1 ) { // set the number of days
			    echo esc_html( $diff . ' days until ticket price increase' ); // what will happen if its more than the number of days
			} else {
				echo esc_html( 'Buy your tickets now' );
			} // enter a note if its too late to book
			?>