drupal/core/modules/datetime_range/datetime_range.module

<?php

/**
 * @file
 * Field hooks to implement a datetime field that stores a start and end date.
 */

use Drupal\Core\Url;
use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function datetime_range_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.datetime_range':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Datetime Range module provides a Date field that stores start dates and times, as well as end dates and times. See the <a href=":field">Field module help</a> and the <a href=":field_ui">Field UI module help</a> pages for general information on fields and how to create and manage them. For more information, see the <a href=":datetime_do">online documentation for the Datetime Range module</a>.', [':field' => Url::fromRoute('help.page', ['name' => 'field'])->toString(), ':field_ui' => (\Drupal::moduleHandler()->moduleExists('field_ui')) ? Url::fromRoute('help.page', ['name' => 'field_ui'])->toString() : '#', ':datetime_do' => 'https://www.drupal.org/documentation/modules/datetime_range']) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Managing and displaying date fields') . '</dt>';
      $output .= '<dd>' . t('The <em>settings</em> and the <em>display</em> of the Date field can be configured separately. See the <a href=":field_ui">Field UI help</a> for more information on how to manage fields and their display.', [':field_ui' => (\Drupal::moduleHandler()->moduleExists('field_ui')) ? Url::fromRoute('help.page', ['name' => 'field_ui'])->toString() : '#']) . '</dd>';
      $output .= '<dt>' . t('Displaying dates') . '</dt>';
      $output .= '<dd>' . t('Dates can be displayed using the <em>Plain</em> or the <em>Default</em> formatter. The <em>Plain</em> formatter displays the date in the <a href="http://en.wikipedia.org/wiki/ISO_8601">ISO 8601</a> format. If you choose the <em>Default</em> formatter, you can choose a format from a predefined list that can be managed on the <a href=":date_format_list">Date and time formats</a> page.', [':date_format_list' => Url::fromRoute('entity.date_format.collection')->toString()]) . '</dd>';
      $output .= '</dl>';
      return $output;
  }
}