It shouldn't be too difficult to make a very simple calendar based on the
Qt calendar widget class. I would like to have that kind of application too. However, since I don't have access to a development environment for the next four weeks, I cannot provide it. And yes, Kobo launcher pbchess. Kobo start menu or fmon could launch the calendar application.
Edit: One could also try building something very basic in the Kobo Start Menu with the cmd line command cal (supported by busybox).