FormatDate process plugin

In my Migration from Joomla to Drupal the format_date plugin failed.

  created:
    -
      plugin: format_date
      from_format: 'Y-m-d TH:i:s'
      to_format: 'Y-m-d\TH:i:s'
      source: created

With the error "Format date plugin could not transform '2012-02-26 11:25:38' using the format 'Y-m-d TH:i:s' for destination 'created'. Error: The date cannot be created from a format."

  created:
    -
      plugin: format_date
      from_format: 'Y-m-d TH:i:s'
      to_format: 'Y-m-d\TH:i:s'
    -
      plugin: date_format_alter
      source: created

If I add strtotime to in a date_format_alter process plugin it works.

class DateFormatAlter extends ProcessPluginBase {
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
      $unixTimestamp = strtotime($value);
      return $unixTimestamp;
    }
}