You can find this feature in : Accounting > Accounting > Action > Lock dates 

You have 3 dates fields :

Here's how they affect the accounting :

- The lock date for non-advisors: The date on which users not belonging to the "Accounting/Invoice Setup" group will no longer be able to post accounting documents for the previous current posting period. E.g.: if you define this date today as 04/20/2020, the users concerned will no longer be able to post accounting documents to the T1 posting period (01/01/2020 - 03/31/2020) as of 04/20/2020. On 04/2020, this date will have to be changed again for Q2. 

- The lock date for all users: Same explanation as the lock date for non-advisers, but this applies to all users, even those belonging to the "Accounting/Invoicing settings" group. 

- Tax lock date: The date on which no user will be able to change an accounting document influencing the previous fiscal period.