How to use to invoice validation to check that no line item date exceeds a specified date

Use case:

In order to standardize monthly reporting we restrict the time periods that may show up on an invoice. This is common for CPA firms. We will bill through the 15th of the month but the invoice may not be generated until the 20th (this is because we are waiting for everyone or fixing timesheet errors). When we create approximately 500 invoices on the 20th we want a control in place that prevents anyone from creating an invoice that has time from a date after the 15th.