[Evolution] Bug in Birthday calendar?

[Rolf Würtz <rolf p wuertz rub de>]

Hi,

I am talking about evolution 3.12.9, the latest version on 
Linux Mint Debian Edition

it is nice that the birthdays of your contacts get displayed
in a special calendar named "Birthdays and Anniversaries".

However, if the person is 67 or older, the birthday gets displayed
the day before at 23:00. 

Is there any reason behind it, am  I doing something wrong, or is
it simply a bug?

Best

Rolf