def posix_to_dt_str(posix):
  """Reverse of str_to_datetime.

  This is used by GCS stub to generate GET bucket XML response.

  Args:
    posix: A float of secs from unix epoch.

  Returns:
    A datetime str.
  """
  dt = datetime.datetime.utcfromtimestamp(posix)
  dt_str = dt.strftime(_DT_FORMAT)
  return dt_str + '.000Z'