time and date

{# Assuming `field_date__value` and `field_date__end_value` are in ISO 8601 format #}

{# Convert the start date #}
{% set start_date = field_date__value|format_date('custom', 'd.') %}
{# Convert the end date #}
{% set end_date = field_date__end_value|format_date('custom', 'd.m.Y') %}

{{ start_date ~ ' - ' ~ end_date }}

use Drupal\Core\Datetime\DrupalDateTime;

/** @var Drupal\Core\Datetime\DateFormatterInterface $date_formatter */
$date_formatter = \Drupal::service('date.formatter');

// Get DateTime object from stored date from a DateTimeFieldItemList field.
$my_date = $node->field_my_date->date;

// Get unix timestamp.
$my_date_timestamp = $my_date->getTimestamp();

// For formatting your DateTime object all other arguments are optional:
// See DateFormatterInterface for built-in options, or use machine name of a date format in config.
$type = 'medium';
// Custom PHP date format if $type="custom".
$format = '';

$formatted = $date_formatter->format($my_date_timestamp, $type, $format, $timezone = NULL, $langcode = NULL);

use Drupal\Core\Datetime\DrupalDateTime;

/** @var Drupal\Core\Datetime\DateFormatterInterface $date_formatter */
$date_formatter = \Drupal::service('date.formatter');

// Get DateTime objects from stored date from a DateTimeFieldItemList field.
$start_date = $node->field_my_date->start_date;
$end_date = $node->field_my_date->end_date;

// For formatting your DateTime object all other arguments are optional:
// See DateFormatterInterface for built-in options, or use machine name of a date format in config.
$type = 'medium';
// Custom PHP date format if $type="custom".
$format = '';

$start_date_formatted = $date_formatter->format($start_date, $type, $format, $timezone = NULL, $langcode = NULL);
$end_date_formatted = $date_formatter->format($end_date, $type, $format, $timezone = NULL, $langcode = NULL);

public static function getDefaultDate() {
  $end_date = new DrupalDateTime();
  $end_date->setTime(23, 59, 59);
  $timezone = new DateTimeZone('UTC');
  $end_date->setTimezone($timezone);
  return $end_date->format('Y-m-d\TH:i:s');
}

default_value_callback: '\Drupal\my_custom_module\Entity\DefaultDateValue::getDefaultDate'

langcode: en
status: true
dependencies: {  }
id: rss_feed_date_format
label: RFC822
locked: false
pattern: 'D, d M Y h:i:s O'