After a short discussion on the IRC and finding out that macOS has no easy way to do that unless I want to install half of the GNU system, I wrote a tool that lets me ask for today in a week et al. which is handy for planning activities when all you know is the distance between a date and the particular activity’s day.

The current iteration of my code seems to work according to my own specs:
https://code.rosaelefanten.org/dateadder/

I hate to have to solve my own problems...