Adding content to Paragraph Date Field Drupal 8
Use the global constant DATETIME_DATETIME_STORAGE_FORMAT to format the date:

// If the time is in a different timezone then adjust it to UTC before you save it
$date-&gt;setTimezone(new \DateTimezone(DATETIME_STORAGE_TIMEZONE));

$paragraph = Paragraph::create([
  ...
  'field_date' =&gt; [
    'value' =&gt; $date-&gt;format(DATETIME_DATETIME_STORAGE_FORMAT),
  ],
]);


Edit

For Drupal >= 8.5: Global constants in datetime.module are deprecated and DateTimeItemInterface has been introduced.