Outlook does have the very nice feature that it integrates birthdays from contacts in the personal calendar. So when you create a new contact with a birthday or change the birthday of a contact this day is displays (and only displayed) as a recuring appointment. With Zimbra and shared contacts / calendars this ets messy quite fast.

Person A creates a contact with a birthday in a shared contact folder. This birthday is displayed in Person A's calendar.
Person B sees the birthday only in the shared contact but not in her calendar AND not in the shared calendar of Person A.

Person B then changes the birthday of said contact to another date. Now the new birthday is display on Person B's calendar but on Person A's calendar the old birthday is still displayed.

Any solution to this conundum?