Check this topic out: Synchronising calendar with Android (free software and services)
I’ve got a Debian distribution running on my Virtual Box. In there I installed ownCloud. There are some guides on the internet. If you have installed ownCloud, you just need to assign a static IP to your Debian distribution, so you can access it from your FP.
If you are not experienced with Linux this sounds quite overwhelming. If you have further questions, don’t hesitate to ask! (Also per PM in German if you prefer.)
Edit: This should probably be in the topic linked above, as it’s not got much to do with the original post.