'Given a date, find that day\'s open and period end (open + offset).'

def calculate_dates(self, dt):

    (period_start, period_close) = self.cal.open_and_close_for_session(self.cal.minute_to_session_label(dt))
    self._period_start = self.cal.execution_time_from_open(period_start)
    self._period_close = self.cal.execution_time_from_close(period_close)
    self._period_end = ((self._period_start + self.offset) - self._one_minute)
