Custom function for Drupal 8 theme (functions) file that returns the URL of the next or previous node of a given content type, based on the value of a given date field's timestamp. Returned values can then be saved as variables in a preprocess node function, for use in the content type's Twig template.

THEMENAME.theme

<?php

/**
 * Custom function to get the URL of the next or previous node of a content type
 *
 * Based on: http://mattkorostoff.com/article/simple-next-previous-nav-for-drupal-8
 *
 * @param  string $content_type    the machine name of a content type to query
 * @param  string $field_name      the machine name of the field to query
 * @param  string $field_timestamp time from the date field (YYYY-MM-DD) to query
 * @param  string $link            either 'prev' or 'next'
 * @return string                  the URL to the next or previous node
 */
function getAdjacentNodeURL( $content_type, $field_name, $field_timestamp, $link ) {
  // setup variables based on $link value
  if ( $link === 'prev' ) {
    $opperator = '>';
    $sort      = 'ASC';
  } elseif ( $link === 'next' ) {
    $opperator = '<';
    $sort      = 'DESC';
  }

  // query database for one older or one newer node of given content type, based on field timestamp
  $query = \Drupal::entityQuery('node');
  $entity_id = $query->condition('type', $content_type)
    ->condition($field_name. '.value', $field_timestamp, $opperator)
    ->sort($field_name . '.value', $sort)
    ->range(0, 1)
    ->execute();
  //kint($entity_id);

  // if $entity_id isn't the youngest or oldest node in the content type
  if ( !empty($entity_id) && is_array($entity_id) ) {
    $node_array = array_values($entity_id);
    $node_id    = $node_array[0];
    // get the alias of the node from the built up path
    $node_url = \Drupal::service('path.alias_manager')->getAliasByPath('/node/' . $node_id);
    // return the URL to be stored in variables for theming
    return $node_url;
  }
}



/**
 * Implements hook_preprocess_HOOK() for node templates.
 */
function THEMENAME_preprocess_node( &$variables ) {
  // Set variables for prev/next links for CONTENTTYPE nodes only
  if ( $variables['node']->getType() === 'CONTNETTYPE' ) {
    // get FIELDNAME date: saved in Drupal in MM-DD-YY format (no time info saved)
    $field_date = $variables['node'] // ContentEntity
      ->get('FIELDNAME')             // FieldItemList
      ->get(0)                       // FieldItem
      ->get('value')                 // TypedData
      ->getValue();                  // string/integer

    // set variables for theming using custom getAdjacentNodeURL function
    $variables['prev_node'] = getAdjacentNodeURL( 'CONTENTTYPE', 'FIELDNAME', $field_date, 'prev' );
    $variables['next_node'] = getAdjacentNodeURL( 'CONTENTTYPE', 'FIELDNAME', $field_date, 'next' );
  }
}