If, for example, you want to issue an invoice for a membership that takes place in July but want the invoice to be in June, you can set the billing period for the invoice to June.

Start by issuing a manual invoice for a member/company by opening their profile, clicking on the Invoices tab, and click Add Invoice.
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This will open the Add Invoice window. Initially, it will be empty. However, if everything has been invoiced for the month, you can use the Pay For option (1) to choose how many months ahead you'd like to issue the invoice.

For example, if you select Pay For 2 months, you'll be able to invoice July memberships while the period of the invoice remains in June.

If there are some items from July that you don't want to invoice, you can simply de-select them using the checkboxes before the line items (2).