Currently, number of days overdue is not a standard merge field that can be used in GIFTS Online templates. We understand the need for this sort of feature and suggest that you create an Idea on our Community page to this effect. Please navigate to https://community.blackbaud.com and log in or create an account. Once logged in, you may navigate to the Idea pages to log the Idea. Remember, the more votes an Idea receives, the more traction it will build with our Product Management team, so please encourage your colleagues to vote on the Idea as well!