At the moment a Challenge either has an open date range (and goes forever) or has a fixed date range.

Many challenges are really recurring challenges, such as a yearly challenge or a monthly challenge.

This suggestion is that the date option allows for turning the challenge into a recurring challenge.

The simplest form would be allowing an extra set of radiobuttons next to the date range (which can't be open) to not recur (default, as now), recur monthly or recur annually. The challenge title would then be suffixed.

Eg.
I create a Christmas Challenge to read 2 books (Number Challenge) and for the date range I might put Dec 15 -> Dec 31, and I select the radio button option of Recur Annually.
This creates a Christmas Challenge (2026) for this year. Next year it would auto create a new one called Christmas Challenge (2027).