You can set per invoice line dates from the Invoice details page, which you can access using the details button on the "Invoices" page in the "OPS" column. For bill lines, you'd do the same from the bill details page which you can access from the "Bills" page.
Note: Once you have set line item specific dates, you can make edits to the date only from the invoice details page, and only if a schedule does not exist for the invoice line item.
To go to the invoice details page (shown above), click "Invoices" then the details icon in the OPS column on the row for the invoice of interest.
Note: If you set a date from the "invoices" summary page, shown below, it will be applied to all line items for that invoice, which you can then override by going into the invoice details page, as shown above.