Show timezone next to datetimes displayed on the Event Espresso event page.

eastern_timezone.php

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

add_filter(
    'FHEE__espresso_list_of_event_dates__datetime_html',
    'my_add_eastern_timezone_display',
    9,
    2
);

function my_add_eastern_timezone_display($html, $datetime){
    if (get_option('timezone_string')) {
        $timezone_string = get_option('timezone_string');
    }
    if ($datetime instanceof EE_Datetime && function_exists('timezone_transitions_get')) {
        $found                   = false;
        $date_time_zone_selected = new DateTimeZone($timezone_string);
        $tz_offset               = timezone_offset_get($date_time_zone_selected, date_create());
        $end_time                = $datetime->end();
        $tr['isdst']             = false;
        foreach (timezone_transitions_get($date_time_zone_selected) as $tr) {
            if ($tr['ts'] > $end_time) {
                $found = true;
                break;
            }
        }
        if ($found) {
            $output = $tr['isdst']
                ? ' EST'
                : ' EDT';
        } else {
            $output = '';
        }
        $html .= '<span>' . $output . '</span>';
    }
    return $html;
}