I should have mention that making the shortcut was to be done in one of the keyboard applications like, textexpander, alfred, keyboard maestro, etc. that easily add the date to you done-keep tag.
If you don't feel like buying a new app to accomplish just this, you should give HammerSpoon a look. It is free and uses Lua. You could easily do it there too.
About the deeper problem, if you don't ask, you don't receive. Explain the feature you want, why you want it, and why you think other people might want it; and it might be implemented if the developer sees the need. Again, that might or not might happen, but this way you can get it done tonight