'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):

    idx = previous_divider_idx(self.market_opens_nanos, dt.value)
    return pd.Timestamp(self.market_opens_nanos[idx], tz='UTC')
