"Final Balance Due 30 Days Before Event Date"?

Hello. I am amazed at the functionality of the Zoho Invoice system. I have been familiarizing myself with it for hours on end, now, and am finding my way around pretty well.

When I execute a contract with a client to DJ their event, the contract specifies that the due date of the final balance (which is the total package fee minus their deposit) is 30 days prior to their event date. I have found a way to configure a custom "Event Date" field in the "Invoices" and "Contacts" preferences. The invoice (and related estimates) now show any Event Date that I specify for a client.

The last piece of the puzzle that I haven't yet been able to figure out is how, if it is possible, to create another new header field that calculates that final balance due date...%EventDate% (if you will) minus thirty (30) days. I know I can go back in and manually put in a custom date, but...

Am I chasing something that doesn't (yet) exist?

Thanks!

Jayson ("DJ Doc")