My mmm calendar shows alway all birthdays, only depending on the day, not the month!

I use 3 calenders, 2 work fine, only the birthday calender is not working properly …

I checked the web, Google and a lot of forums, but it seems that it must my personal problem, because nobody else is having that issue.

What can I do wrong to get that result?

To be mor precise:

Jeffs birthday is January 2nd, and mmm shows me Jeff every month. And this is the case with all entries of that calender!

Regards, Hai