Migrating calendars from horde to zarafa

Recently I had to migrate a bunch of calendars from horde to zarafa. At the time of this writing (spring 2013) caldav access to horde is still just on the roadmap of horde. So basically, I had to log in for every users and export their calendars to ical files by hand. I named them username.ical.
Fortunatly, importing into zarafa can be automated. After importing a file, I realized that recurring events where off by 2 hours. In the ical files, the times were correct, but after importing them, recurring events startet and ended two hours later. So I had to modify the ical files. I did that with the following python script: Note that I used python icalendar 3.3, which exposed this issue. I made a temporary fix (see link), but with icalendar 3.4 and later this should be resolved.
So after the recurrence timing had been corrected, I imported all ical files with