<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _FormatDate(self, event):
 """Formats the date. Args: event (EventObject):
 event. Returns: str: date field. """

    # TODO: preserve dfdatetime as an object.
    # TODO: add support for self._output_mediator.timezone
    date_time = dfdatetime_posix_time.PosixTimeInMicroseconds(
        timestamp=event.timestamp)

    year, month, day_of_month = date_time.GetDate()
    try:
      return '{0:04d}-{1:02d}-{2:02d}'.format(year, month, day_of_month)
    except (TypeError, ValueError):
      self._ReportEventError(event, (
          'unable to copy timestamp: {0!s} to a human readable date. '
          'Defaulting to: "0000-00-00"').format(event.timestamp))

      return '0000-00-00'