Billable Time: adding to existing invoices

After creating an invoice via the billable time module, I sometimes have to add another billable time entry. There is an option to invoice it to an existing invoice (or write off etc). This doesn’t work, even if no payment has been paid and logged against that invoice- the billable time entry remains uninvoiced and that billable time item doesn’t appear on the invoice.

Bug or have I messed up somehow?

There are number of reasons.

One of the reasons is that billable time cannot be linked to an invoice because billable time is dated after the sales invoice. Could it be this?

I think you are right about the dating issue.