custom zip

    @staticmethod
    def __zip_prev_with_new(prev_groups, new_groups):
        from itertools import ifilter

        def drop_tz_unsafe(date):
            return date.replace(tzinfo=None)

        left = map(list, prev_groups)
        right = map(list, new_groups)
        zipped = []

        while len(left):
            old_group = left.pop()
            start_date = drop_tz_unsafe(old_group[0].billing_period_start_date)
            new_groups_iter = ifilter(
                lambda (idx, grp): drop_tz_unsafe(grp[0].billing_period_start_date) == start_date,
                enumerate(right)
            )
            new_group_idx, _ = next(new_groups_iter, (-1, None))
            pair = (old_group, right.pop(new_group_idx)) if new_group_idx >= 0 else (old_group, [])
            zipped.append(pair)

        while len(right):
            zipped.append(([], right.pop()))

        return zipped