Allow reservations only one year in advance.

limit-bookings-to-year-in-advance.php

<?php

add_action( 'wp_head', function () { ?>
<script>

    jQuery( document ).on( 'jet-booking/init', () => {

        window.jetBookingState.filters.add( "jet-booking/input/config", modifyBookingConfig );
        window.jetBookingState.filters.add( "jet-booking/calendar/config", modifyBookingConfig );
        
        function modifyBookingConfig( config ) {
            
           config.endDate = moment().add(1, 'y').subtract( 1, 'd' ).format( config.format );
           return config;
            
        }

    } );
    
</script>
<?php } );