datetime.scratch.php

<?php

/**
 * Random Datetime stuff
 */

function example_field_modify() {
  $timezone = new DateTimeZone('Asia/Kolkata');
  new DateTime($datestring, $tz);
  // Create an entity with a date that we'll enter in Amercia/New York time.
  $date_format = DateFormat::load('html_date')->getPattern();
  $time_format = DateFormat::load('html_time')->getPattern();
  $field['value'][0]['date'] = $start_date->format($date_format);
  $field['value'][0]['time'] = $start_date->format($time_format);
  $field['value'][0]['timezone'] = 'America/New_York';

  $dt = new DateTime($date);
  $offset = new DateInterval('PT' . date('I', strtotime($date)) . 'H');
  $dt = $dt->sub($offset);
  return date_iso8601($dt);
  // We overwrite the values with the exact same time but with the timezone
  // set to the timezone the user told us they were using when entering the
  // dates. We do this here, and then DateRangeWidgetBase converts these
  // values into UTC for storage. This makes displaying the dates correctly
  // simple. We use the UTC stored date time plus the timezone stored.
  if ($values[$delta]['value'] instanceof DrupalDateTime) {
    $values[$delta]['value'] = new DrupalDateTime($values[$delta]['value']->format(DATETIME_DATETIME_STORAGE_FORMAT), $timezone);
    $values[$delta]['end_value'] = new DrupalDateTime($values[$delta]['end_value']->format(DATETIME_DATETIME_STORAGE_FORMAT), $timezone);
  }
  
  return [
    'value' => [
      'date' => $start_date->format($date_format),
      'time' => $start_time->format($time_format),
    ],
    'end_value' => [
      'date' => $end_date->format($date_format),
      'time' => $end_time->format($time_format),
    ],
    'timezone' => $timezone,
  ];
}

// Instantiate with timestamp.
DrupalDateTime::createFromTimestamp($comment->getCreatedTime());
// Reset node cache after bulk edits.
\Drupal::entityManager()->getStorage('node')->resetCache($nids);
// Format and re-instantiate a DrupalDateTime instance.
$formatted = $ddt->format(DATETIME_DATETIME_STORAGE_FORMAT, ['timezone' => $tz]);
return new DrupalDateTime($formatted);