Which dates must be completed in Mobility as arrival and departure date of my stay (research or courses) abroad?

If you follow courses, you must look at the course schedule of the host institution.

If you will do your master thesis abroad, you choose the dates together with the host institution/lab. The faculty does not impose any dates. You usually leave in January or February and return at the end of June for your master thesis defense.

If you also do your project proposal abroad (mostly BMW students), you can leave earlier.

If your exchange period mainly takes place during the first/second semester, than that is the semester you must indicate in Mobility-Online. A full academic year is only applicable if you leave for a whole year and you do not take up any courses here anymore.