def timestamp_str(self, tz: datetime.tzinfo | None = None, timespec: str = 'auto') -> str:
        """Return a string for the calibration timestamp.

        Args:
            tz: The timezone for the string. If None, the method uses the
                platform's local timezone.
            timespec: See datetime.isoformat for valid values.

        Returns:
            The string in ISO 8601 format YYYY-MM-DDTHH:MM:SS.ffffff.
        """
        dt = datetime.datetime.fromtimestamp(self.timestamp / 1000, tz)
        dt += datetime.timedelta(microseconds=self.timestamp % 1000000)
        return dt.isoformat(sep=' ', timespec=timespec)
Return a string for the calibration timestamp.

        Args:
            tz: The timezone for the string. If None, the method uses the
                platform's local timezone.
            timespec: See datetime.isoformat for valid values.

        Returns:
            The string in ISO 8601 format YYYY-MM-DDTHH:MM:SS.ffffff.
        
cirq-google/cirq_google/engine/calibration.py