Date math in update/change field value

The options for dates are interesting and helpful but I'd like the option to "do math" in here. For example, I'm storing the dates people buy something. Their access lasts for a year. If they purchase a 1-year extension, I want to add 365 days to their purchase date ... which is completely independent of what day today is. Would be great to do here, as we can with merge fields in the email/page text, [PRW purchase date]+365 ... seems now like I will have to put them in a wait sequence that waits until the current day, and then updates the day to be 1 year from now. Doable, just not as elegant as giving us the option of math right here.