Booking bar form with date validation

<?php
/**
 * Booking bar: shortcode + admin-post handler.
 *
 * @package Acme_Theme
 */

defined( 'ABSPATH' ) || exit;

function acme_booking_bar(): string {
	ob_start();
	?>
	<form class="acme-booking" method="post" action="<?php echo esc_url( admin_url( 'admin-post.php' ) ); ?>">
		<input type="hidden" name="action" value="acme_booking_search">
		<?php wp_nonce_field( 'acme_booking_search', 'acme_nonce' ); ?>
		<label>
			<?php esc_html_e( 'Check in', 'acme-theme' ); ?>
			<input type="date" name="check_in" min="<?php echo esc_attr( wp_date( 'Y-m-d' ) ); ?>" required>
		</label>
		<label>
			<?php esc_html_e( 'Check out', 'acme-theme' ); ?>
			<input type="date" name="check_out" required>
		</label>
		<label>
			<?php esc_html_e( 'Guests', 'acme-theme' ); ?>
			<input type="number" name="guests" value="2" min="1" max="8" required>
		</label>
		<button type="submit"><?php esc_html_e( 'Check availability', 'acme-theme' ); ?></button>
	</form>
	<?php
	return (string) ob_get_clean();
}
add_shortcode( 'acme_booking_bar', 'acme_booking_bar' );

function acme_booking_handle(): void {
	$back = wp_get_referer() ?: home_url( '/' );

	if ( ! isset( $_POST['acme_nonce'] ) || ! wp_verify_nonce( sanitize_text_field( wp_unslash( $_POST['acme_nonce'] ) ), 'acme_booking_search' ) ) {
		wp_safe_redirect( add_query_arg( 'booking_error', 'nonce', $back ) );
		exit;
	}

	$in     = sanitize_text_field( wp_unslash( $_POST['check_in'] ?? '' ) );
	$out    = sanitize_text_field( wp_unslash( $_POST['check_out'] ?? '' ) );
	$guests = min( 8, max( 1, absint( $_POST['guests'] ?? 1 ) ) );

	$d_in  = DateTimeImmutable::createFromFormat( '!Y-m-d', $in );
	$d_out = DateTimeImmutable::createFromFormat( '!Y-m-d', $out );
	$today = new DateTimeImmutable( 'today', wp_timezone() );

	if ( ! $d_in || ! $d_out || $d_in < $today || $d_out <= $d_in ) {
		wp_safe_redirect( add_query_arg( 'booking_error', 'dates', $back ) );
		exit;
	}

	wp_safe_redirect( add_query_arg( array( 'in' => $in, 'out' => $out, 'guests' => $guests ), home_url( '/rooms/' ) ) );
	exit;
}
add_action( 'admin_post_nopriv_acme_booking_search', 'acme_booking_handle' );
add_action( 'admin_post_acme_booking_search', 'acme_booking_handle' );

document.querySelectorAll('.acme-booking').forEach((form) => {
	const [inEl, outEl] = form.querySelectorAll('input[type="date"]');
	inEl.addEventListener('change', () => {
		// Check out must be at least one night after check in.
		const next = new Date(inEl.value);
		next.setDate(next.getDate() + 1);
		outEl.min = next.toISOString().slice(0, 10);
		if (outEl.value && outEl.value < outEl.min) outEl.value = outEl.min;
	});
});