Maybe. I'm working on a date field as well and managed to get the value to store.

What are your storage settings? Please add a screenshot of your calendar field settings.

Thanks for your answer. I have posted a link to a screenshot from the settings in my question ;) here it is again: 

http://picload.org/view/cloodog/1.png.html

Hello again, 

I was some hours overtime yesterday. :)

Have you experimented with different storage settings?

You should not use storage Custom - delete the field and create it again and set it to storage Standard-Article-yourfieldname

Thank you Klas. Now it works :)

In your link it says that "Standard"-fields be used in 99% of cases. If you create a new field the storage format "custom" is pre selected. Now all my fields are saved as custom (only my calendar field ist now "standard"). I still do not understand really the difference between standard and custom. My english is not good sorry :) 

So my last question is... the storage format custom for my fields... is that a disadvantage?

Thanks guys ;)