To prevent people from inadvertently posting to a period that is not open, either in the past or future, you should set up a posting date range.

Go to Financial Management > Setup > General Ledger Setup.

In the Allow Posting From field, put in the earliest date you want to allow posting.  In the Allow Posting To field, put in the latest date you want to allow posting.  Navision does not prevent users from posting to closed periods, this posting date range prevents inadvertent posting.  If these fields are blank, users can post to any period.

You can further define the posting date ranges by setting up user ranges, which allow specific date ranges for particular users, for instance, supervisors or managers who need to post transactions outside of this range,.  You do not need to set up a range for everyone, only the exceptions, everyone else will have the system default.

Go to Administration > Application Setup > Users > User Setup.

Select the users whose range you want to change from the default, and put in Allow Posting From and Allow Posting To dates.