Time and date stamp

Hi

Was wondering if you would be able to help me with a formula.

I have a worksheet that is filled in by users as they input invoices.

In Cell A8 i have written the following formula:

If(B8="","",NOW())

The idea is that if the user types an invoice number into B8 it will record the time and date they entered the record. This formula works for the first record but after i copy the formula down through column A if something is entered into B9 it will update A9 as also A8 which will distort the figures. Is there any way to stop this from happening?

Hope this makes sense, if not please let me know.

Many thanks

Tony