def serialize_iso(attr, **kwargs):
        """Serialize Datetime object into ISO-8601 formatted string.

        :param Datetime attr: Object to be serialized.
        :rtype: str
        :raises: SerializationError if format invalid.
        """
        if isinstance(attr, str):
            attr = isodate.parse_datetime(attr)
        try:
            if not attr.tzinfo:
                _LOGGER.warning(
                    "Datetime with no tzinfo will be considered UTC.")
            utc = attr.utctimetuple()
            if utc.tm_year > 9999 or utc.tm_year < 1:
                raise OverflowError("Hit max or min date")

            microseconds = str(attr.microsecond).rjust(6,'0').rstrip('0').ljust(3, '0')
            if microseconds:
                microseconds = '.'+microseconds
            date = "{:04}-{:02}-{:02}T{:02}:{:02}:{:02}".format(
                utc.tm_year, utc.tm_mon, utc.tm_mday,
                utc.tm_hour, utc.tm_min, utc.tm_sec)
            return date + microseconds + 'Z'
        except (ValueError, OverflowError) as err:
            msg = "Unable to serialize datetime object."
            raise_with_traceback(SerializationError, msg, err)
        except AttributeError as err:
            msg = "ISO-8601 object must be valid Datetime object."
            raise_with_traceback(TypeError, msg, err)
Serialize Datetime object into ISO-8601 formatted string.

        :param Datetime attr: Object to be serialized.
        :rtype: str
        :raises: SerializationError if format invalid.