code: def previous_open(self, dt):
          
          idx = previous_divider_idx(self.market_opens_nanos, dt.value)
          return pd.Timestamp(self.market_opens_nanos[idx], tz=UTC)
docs: 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.