How to setup a task deferred for a specific duration to a specific date

This question comes from one of my real case, here I use credit card to explain the issue.
Let’s sat my credit card’s bill date is every 25th of month. But the rule is “i need to pay it within 20 day’s after the bill date”. But 20 days after every 25th of month will not become a fixed date for every month.

Some cases is similar, such as, I need to check the status of something after a duration of specific date, but the final date is not fixed, only the duration is fixed.

Is there a way in omnifocus can setup the task like “due a specific duration to a fixed date”? Or any trick to implement the same purpose?