Tax reporting over a period requires you to report balances for asset positions you hold as well as any assets held before the period and carried over.

Just nominate the financial period or manually enter start and end date and CHAINOMETRY will calculate the opening and closing asset quantities you own by the period chosen.

In order to do this on an ongoing basis we need to keep your historical data as you may hold on to assets for longer that a single period. There is no concept of 'rollover' except that all historical data is processed and balances are calculated on the fly when you generate a report.