'Given a dt, return the previous exchange minute.
Raises KeyError if the given timestamp is not an exchange minute.
Parameters
dt: pd.Timestamp
The dt for which to get the previous exchange minute.
Returns
pd.Timestamp
The previous exchange minute.'

def previous_minute(self, dt):

    idx = previous_divider_idx(self._trading_minutes_nanos, dt.value)
    return self.all_minutes[idx]
