Reporting periods can be updated at any time and will not affect any existing transactions posted to those periods. Reporting periods set the dates in which transactions can be created. You cannot process any transactions without a period that includes the current date. 

Reporting periods also appear as date ranges to select from on most Clear Books reports. Try not to have overlapping periods and for clarity keep the periods in chronological order. 

To prevent transactions from being created in older periods you can lock the period.

Locking accounting periods

Reporting periods are listed on the Settings > Accounting > Periods > Reporting Periods menu.

If you have more than 1 reporting period you can make changes by deleting the period using the delete icon, shown in the Options column.  Deleting a period does not remove any transactions. Once the period is deleted, you can re-add with the correct dates. 

If you only have 1 reporting period then you can edit the date range using the edit icon under the Options column.