https://issues.roundup-tracker.org/issue1169748 implements a custom action that returns a calendar file given urls like:
http://localhost:8917/demo/issue1?@action=calendar http://localhost:8917/demo/issue?@filter=status,assignedto&status=-1,1,2,3,4,5,6,7&assignedto=username_or_id&@action=calendar
Best to download the .py file from:
https://issues.roundup-tracker.org/file320/calendar.py
and see the issue for details on how to install.