Unless I'm mistaken, birthday fields in Zimbra 6.x are now a free format text field. ie. users may enter "17/12/1974", "12/17/1974" or "hello world" in the web UI.
Further, the picker generates a string in the format of whatever the current locale happens to be at the time of data entry eg mm/dd/yyyy.
This means that SOAP clients (like the zindus addon) have no sure way of parsing a birthday field. or, if it happens to be parsable, distinguishing whether "11/12/1982" is a date in November or December.
This is in contrast to 5.x in which the UI was a date picker that consistently generated yyyy-mm-dd.
If I have somehow got the wrong end of the stick here, would someone please let me know?
Otherwise the zindus addon will not sync the Zimbra birthday field, because while Zimbra itself may cope with arbitrary text in a birthday field, neither Thunderbird nor Google will.