a. There is not a date entry shortcut.
b. I don’t think you have.
I have experimented with weird date goals like this, and talked about it in the forum: Better option for oldest-item do less goals, today?. Please read that. I still use my epoch date goals.
I really think that when we figure out a good way to do this, Beeminder should have this built in. I would love to have more people experimenting with this, and if you try it, or even decide not to, please let us know how it goes. If we can build a solid set of suggestions of changes to make this usable and easy for folks, I’m certain that will be super helpful for Beeminder.