Not sure what to suggest re the invoice number - perhaps in form current event, make TheField = ID + 1000 as I'd say it's possibly a necessary value in a table but I don't see a need to store a date calculation. If this is for a printed invoice, the report control would be the TheDateYouNeed + 90, and that's what gets printed. If it's for display in a form too, the form field should be TheDateYouNeed + 90 (or use DateAdd function for either). Storing the date doesn't seem necessary in this case and it's usually ill advised anyway.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.