Given a dt, returns the previous open.

        Parameters
        ----------
        dt: pd.Timestamp
            The dt for which to get the previous open.

        Returns
        -------
        pd.Timestamp
            The UTC imestamp of the previous open.

def previous_open(self, dt):
        """
        Given a dt, returns the previous open.

        Parameters
        ----------
        dt: pd.Timestamp
            The dt for which to get the previous open.

        Returns
        -------
        pd.Timestamp
            The UTC imestamp of the previous open.
        """
        idx = previous_divider_idx(self.market_opens_nanos, dt.value)
        return pd.Timestamp(self.market_opens_nanos[idx], tz=UTC)