Most businesses usually align to the standard tax year for accounting purposes - which is between 6th April - 5th April each year. However, if your business uses different accounting dates (e.g. 1st January - 31st December), then you can log these custom dates in your settings and the sole trader tax estimation tool will adjust your estimations accordingly.
If you decide to change your accounting period, then this will apply to all past and future tax estimates. Changing your accounting period part way through the tax year, could result in double counting or missed transactions.
If your custom accounting period spans two standard tax years (6th April - 5th April), your entire custom accounting period will use the tax rates of the latest standard tax year. It’s possible that for future tax years, tax rates may not have been announced by HMRC – and in this instance, the tax estimation will use the existing rates until the new rates are confirmed.