To export certain dates from TISS, namely from the courses you're interested in (not all of them in your favorites), follow these steps:
1. Create categories. The courses of which you want to export the lecture lessons must not be in the same category as the courses where you do not want the lecture lessons to appear in your calendar. You can have multiple categories of both (courses of which you want to export the lecture lessons and courses where you don't), they just must not overlap.
2. Check the options of every category: If you want the lecture dates of the category to appear in your calendar, make sure to check the last checkbox in the Calendar-category with the title "Show appointments". If you don't, uncheck it!
3. Click on "Calendar" on the left side in the "Cockpit" - category. At the end of the page, you can generate a Token, namely a URL, which you can import into external calendars, like into Google calendar.