Returns
        -------
        sessions : DatetimeIndex
           All session labels (unioning the range for all assets) which the
           reader can provide.

def sessions(self):
        """
        Returns
        -------
        sessions : DatetimeIndex
           All session labels (unioning the range for all assets) which the
           reader can provide.
        """
        return pd.to_datetime(
            reduce(
                np.union1d,
                (reader.dates for reader in self._readers.values()),
            ),
            utc=True,
        )