Hi,
When using the old JSLink installation method in a calendar you must manually add a link to a file “/SPJS/DFFS/JSLink/DFFS_CalendarLoader.html” in the forms (in a Content Editor Web Part). This is because SharePoint does not support JSLink in calendars.
Unfortunately this installation method is not described in the user manual anymore because it has been deprecated. You should use the DFFS “Local Installer” instead: https://spjsblog.com/dffs/dffs-installation-manual/#Step-by-step
Alexander