(Drupal 8.x) Example code: Add Drupal events to Google calendar with AddToCalendar. https://addtocalendar.com

mytheme.libraries.yml

add-to-calendar:
  css:
    component:
      '//addtocalendar.com/atc/1.5/atc-style-menu-wb.css': {type: external}
  js:
    '//addtocalendar.com/atc/1.5/atc.min.js': { type: external, minified: true }

mytheme.theme

<?php 

/**
 * Preprocesses variables for node--event.html.twig.
 *
 * This is an untested, simplified version of production code that used 
 * paragraph entities to store date information, with multiple date ranges 
 * possible per node. This code assumes one date range consisting of two
 * datetime fields.
 */
function mytheme_preprocess_node__event(&$vars) {
  $node = $vars['node'];

  // Getting ready to populate properties for AddToCalendar.
  $vars['#attached']['library'][] = 'mytheme/add-to-calendar';
  $atc = [
    'atc_date_start' => '',
    'atc_date_end' => '',
    'atc_timezone' => 'America/Edmonton',
    'atc_title' => $node->getTitle(),
    'atc_description' => '',
    'atc_location' => '2 Sir Winston Churchill Square, Edmonton, Alberta T5J 2C1',
    'atc_organizer' => 'Art Gallery of Alberta',
    'atc_organizer_email' => '[email protected]',
  ];

  $atc['atc_description'] = $node->get('field_event_teaser')->view('teaser');
  $atc['atc_description'] = render($atc['atc_description']);
  $atc['atc_description'] = trim(strip_tags($atc['atc_description']));

  $start = $node->field_event_start_date->getValue();
  $date = $start->getValue();
  $date = new DrupalDateTime($date[0]['value'], new DateTimeZone('UTC'));
  $atc['atc_date_start'] = $date->getTimestamp();
   
  $end = $node->field_event_end_date->getValue();
  $date = $end->getValue();
  $date = new DrupalDateTime($date[0]['value'], new DateTimeZone('UTC'));
  $atc['atc_date_end'] = $date->getTimestamp();

  foreach ($atc as $k => $v) {
    $build[$k] = [
      '#type' => 'html_tag',
      '#tag' => 'var',
      '#value' => $v,
      '#attributes' => [
        'class' => $k,
      ],
    ];
  }
  
  $wrapper = [
    '#type' => 'html_tag',
    '#tag' => 'span',
    '#value' => '<a class="atcb-link">Add to Calendar</a><var class="atc_event">' . render($build) . '</var>',
    '#attributes' => [
      'class' => [
        'addtocalendar',
        'atc-style-menu-wb',
      ],
    ],
  ];

  // You can append this to any field.
  $vars['content']['field_event_start_date'][$i]['#suffix'] = render($wrapper);
}