AP Invoices that are posted to the wrong period must be handled very carefully. The reason is that AP invoice entry affects the Open AP reports that are run at the end of each period. These AP reports are used to balance the AP sub-ledger (individual invoice balances) to the AP general ledger account. The AP reports show invoice balances as of a specific moment in time (month end) and the reports are not reproducible once processing occurs in the next period. If an AP transaction (invoice entry, check payment) is posted to a prior period, the transaction will be reflected in the GL postings for the period, but the change will not be reflected on the report that was produced at the end of the period. To make matters worse, if the prior period was closed, and the AP invoice affects income statement accounts (like it usually does), the period may have to be reopened and then closed again to reflect the change caused by the invoice or payment entry. Generally if an invoice is posted to a prior period and the period has been closed, an offsetting invoice can be posted to the prior period to cancel the effect of the first mis-posted invoice (the invoice and the offsetting invoice are then applied to each other to close them both). The invoice can then be reposted to the correct period. The same procedure can also be used to correct invoices that are posted to incorrect future periods. Posting invoices to a prior period is strongly discouraged but if it is done, you should manually note any change (in writing) on any month end reports that are affected by the posting, and re-close the period (if applicable).