Why should this be included in the repository? Because it's a highly customizable applet that integrates google calendar, remember the milk and Outlook. It has a Lite ($0) version and a Pro version (~$10). Currently packaged for Ubuntu. The generic version has some dependency problems (libwebkitgtk-1.0.so.0).
Is it Open Source? No :(
How many users do you anticipate will use this software? Everyone can use this nice piece of software, so potentially can be useful for all.
Link to tarball : http://www.rainlendar.net/download/Rainlendar-Lite-2.13.1-amd64.tar.bz2
Given we have no plans on flatpaking or snapping this application, closing as wontfix. I'd suggest reaching out to the respective developers about supporting either of those distribution and container models, or taking on the initiative.