Example of how to add text to the datetime list.

example.php

<?php //Please od not include the opening PHP tag if you already have one.

add_filter(
  'FHEE__espresso_list_of_event_dates__datetime_html',
  'add_text_after_datetime_list', 
  20, 
  2
);
function add_text_after_datetime_list( $html, $datetime ) {
  $html .= '<br><span class="ee-datetimes-timezone">';
  $html .= 'Pacifict Standard Time (PST)';
  $html .= '</span>';
  return $html;
}