Hi Lakshmipathi,
I have checked couple of Excise Invoices and noticed that system updates the "Excise Document Date" (J_1IEXCDAT) as per the "Posting Date" which is available in the Excise Invoice in the table J_1IEXCHDR . Correct me if I am wrong.
In our case we always refer a billing document for creating an Excise Invoice.
with regards,
Dayanand