iCal is an Internet Engineering Task Force (IETF) standard for calendaring and scheduling. http://www.ietf.org/rfc/rfc2445.txt
You can export your tasks to the iCal format using
planner-ical. Add (require 'planner-ical) to your
~/.emacs. Then you can use M-x planner-ical-export-page
to display the iCal equivalent of tasks on a page, which you can then
save to a file.
To export today's tasks to a file in your publishing directory, add the following to your ~/.emacs.
(planner-ical-export-file
(planner-today)
(expand-file-name
"tasks.ics"
(muse-style-element :path (car (cddr (muse-project planner-project))))))