How can I set the end date of a membership to be calculated from birth date?
We created an extension that added this functionality.

Edit:
Unfortunately the extension is not publicly available as it is mixed in with a whole collection of custom stuff we needed to do for this client. Also our requirements changed so we were no longer working off birth date but still setting the end date based on a field completed whilst completing the form.

I am not the coder but as far as I can understand we created a function to set end date to the value we then calculated from the field of interest.