I haven’t used it in probably a decade, but GeekTool[0] lets you display terminal output on your desktop. I imagine it could be setup where a script runs periodically to generate the life calendar output, and the applet on the desktop would then update.
Still might feel a bit janky, and not sure if it’s the look you’re going for, but it’s another avenue if you’re looking for options.
[0] https://www.tynsoe.org/geektool/