Although I don't think this is the best way to do it. Maybe:
Invoice Number = Customer ID & "-"& getasnumber( Purchased Date )
Why not just have a sequential serial number incrementing by 1 on creation acting as your invoice number?
Alternatively if you change your calculation to be Invoice Number = Customer ID & Year ( Purchased Date ) & Month ( Purchased Date ) & Day ( Purchased Date ) it will not add the numbers together, but put them in a string, but then what happens if a customer needs two seperate invoices on the same day? your invoice number would still not be unique.
I was so fixated on math that I didn't consider using a string. Doh! For post analysis of the data this is prefect.
Thanks for the suggestions!