def get_headers(self, date_headers, from_date, to_date, trunc):
        """Adjust date headers & get range headers."""
        date_headers = list(date_headers)

        # Earliest date should be no earlier than from_date.
        if date_headers and date_headers[0] < from_date:
            date_headers[0] = from_date

        # When organizing by week or month, create a list of the range for
        # each date header.
        if date_headers and trunc != 'day':
            count = len(date_headers)
            range_headers = [0] * count
            for i in range(count - 1):
                range_headers[i] = (
                    date_headers[i], date_headers[i + 1] - relativedelta(days=1))
            range_headers[count - 1] = (date_headers[count - 1], to_date)
        else:
            range_headers = date_headers
        return date_headers, range_headers
Adjust date headers & get range headers.