To create an extended accounting period you need to go to client data / edit common data / period / edit.

You need to enter the year end (dd/mm) and then on the right hand side enter the number of days in the period. If you click the option next to the number of days box you can use the calendar to calculate the days. On the face of the accounts it will show the number of days to the year end, this is required for iXBRL tagging purposes.