Would like to be able to set a relative date in a custom date field

As an admin, I have a customer who wants to add a custom date field to their screens, which is defaulted to "three weeks from the day the issue is entered". So, I would like to be able to set a default value at something like "today + 21" or "creation date + 21". Even better would be the ability to set it to fall on a weekday - e.g. "first weekday after (creation date + 10)".

Issue #16366 seems related insofar as it's asking for relative dates, but that's about bulk changing the due date, not setting the default.