You could use this relationship between Invoices and Orders to match to all Orders records for a given client for a specified range of dates:
Invoices::_fkClientID = Orders::_fkClientID AND
Invoices::StartDate > Orders::OrderDate AND
Invoices::EndDate < Orders::OrderDate
You can double click a relationship line in Manage | Database | relationship to add more match field pairs and to change = operators into other operators such as the inequality operators specified for this relationship.
Many thanks - that is really helpful!