Hello, 

Is it possible for me to update the closing date field for an opportunity by setting a different date field?

For instance, my company has quotations that last 30 days, 45 days, and 90 days. If we set the stage to a 30 day Quotation, and set a custom field "Quote Date" as the day we send the quotation, is it possible to update the Closing Date automatically to 30 days after the Quote Date? 

I'm aware I could use Execution Date, but not all quotes go in the day they're sent so it wouldn't be ideal. 

Is this something I can do with a workflow rule, or do I need to write a function for it?