Serialize wrapped datetime.timedelta instance to a string the
        with the following format:
            [DAYS]d [SECONDS]s [MICROSECONDS]us

def to_json(cls, obj):
        """
        Serialize wrapped datetime.timedelta instance to a string the
        with the following format:
            [DAYS]d [SECONDS]s [MICROSECONDS]us
        """
        return "{0}d {1}s {2}us".format(
                obj.days, obj.seconds, obj.microseconds)