I have found an older invoice from december my vat period closed end december do I log it into december or my current vat period

Hello Pubsignsuk, 

Welcome to the Community page, 

If the invoice was for December you would log it as a December one, then on your tax return, it will show up as an exception so that it still gets the vat accounted for. You can go to the vat detail report to look at the historical vat to check the transactions in there, if not enter the transaction and it becomes an exception for you.