This allows you to change the timezone used for the iCal data, if a custom field value is present it will be used to set the timezone on the datetime. Useful for sites that have events in different timezones

ical_offset_example.php

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

add_filter(
    'FHEE__EED_Ical__download_ics_file_ics_data',
    'my_custom_ical_timezone_output_filter',
    10,
    2
);
function my_custom_ical_timezone_output_filter(
    $ics_data, 
    $datetime
) {
    $event = $datetime->event();
    if($event instanceof EE_Event) {
        $timezone = $event->get_post_meta('event_timezone', true);
        if(! empty($timezone)) {
            $datetime->set_timezone($timezone);
            $ics_data['DTSTART'] = date(
                EED_Ical::iCal_datetime_format, 
                EEH_DTT_Helper::get_timestamp_with_offset(
                    $datetime->start(),
                    $timezone
                )
            );
            $ics_data['DTEND'] = date(
                EED_Ical::iCal_datetime_format, 
                EEH_DTT_Helper::get_timestamp_with_offset(
                    $datetime->end(),
                    $timezone
                )            
            );
        }
    }
    return $ics_data;
}