I just discovered this might not be needed because I can type "1 month" or "1 year" in the Calendar Picker's date field:

https://www.rememberthemilk.com/new/

Quick date picker
Because less typing = more free time! When entering dates in words (e.g., “2 weeks”), you can now auto-complete your choice or select from frequently used dates.

Correction: That should say "in the Due Date field," not the Calendar Picker's date field.

This can be useful when you complete a form that you need to file every year with a government, like your income tax return or to pay an annual license fee. Usually their is a deadline date and a start date. With your income tax it is after you receive you earning confirmation and the deadline is a few months later.

This feature has already been implemented.
For dates, see https://www.rememberthemilk.com/help/answer/basics-basics-dateformat
For repeat intervals, see https://www.rememberthemilk.com/help/?ctx=basics.basics.repeatformat
When in doubt, try it!