Prints a message regarding the duration of the event. For Event Espresso 4

event_duration.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_action( 'AHEE__ticket_selector_chart__template__before_ticket_selector', 'ee_print_event_duration', 10 );
function ee_print_event_duration( $event ) {

    $datetimes = $event->datetimes();
    foreach($datetimes as $datetime){
        if ( $datetime instanceof EE_Datetime ) {
            $start = $datetime->get_raw('DTT_EVT_start');
            $end = $datetime->get_raw('DTT_EVT_end');
            $diff = $end - $start;
            $hours = $diff / 3600;
            echo '<div><strong>This is ' . $hours . ' hours in duration</strong></div>';
        }
    }
}