A pain quite frankly. Is this likely to be for a lot of invoices?
Let's say you've invoiced £1200 on the 1st of Jan. On that same day you would create a journal, you'd DR sales with £1100 and CR def revenue with £1100. That would leave your Jan P&L showing a sale of only £100 i.e. 1/12 of the total invoice and the other £1100 would be sitting in the def revenue account. You'd then need to create 11 further monthly journals from 1st Feb onwards to move £100 back from def revenue into sales (you'd DR def rev this time and CR sales). You P&L would then show £100 sales each month and the def revenue account would slowly zero off.
Unfortunately there's no automatic process to achieve the above so you're going to have to create each one manually - a huge time sink if you've got lots of invoices. The best you can do is 'copy' each journal and just amend the date to the following month.
The only other way to do it would be via a recurring monthly invoice that automatically 'paid' itself to a dummy bank account called deferred revenue - it would clog up your invoice list though which can be a slight pain too.