http://demo.sogo.nu/SOGo seems to be currently on version 3.0.1 (according to what is shown in preferences).

On my env I can reproduce the problem if I set the birthday to a date between e.g. 25-sep-83 and 30-oct-83 or 30-sep-05 and 27-oct-85. Dates between these sample dates results in a BDAY one day earlier.

User's timezone: Europe/London

The reproducibility of the problem depends the timezone setting of.

e.g.:
SOGoTimeZone=Europe/Vienna
User Prefernces=Europe/Vienna
Browser/Client=Europe/Vienna

Setting the birthday to 2016-03-13:
The browser returns an integer which is converted in UIxContactEditor.m->setAttributes:

date = [NSCalendarDate dateWithTimeIntervalSince1970: seconds];

to this date: 2016-03-13 00:00:00 +0100

The resulting value for BDAY is 20160313. -> OK

Setting the birthday to 1985-10-26:
The browser returns an integer which is converted in UIxContactEditor.m->setAttributes:

date = [NSCalendarDate dateWithTimeIntervalSince1970: seconds];

to this date: 1985-10-25 23:00:00 +0100

The resulting value for BDAY is 19851025. -> Wrong

I hope this finding helps to fix the problem.

==

Another timezone related issue might be in NGVCard+SOGo.m->birthday:

I think to have consistent results this should be changed from

  date = [NSCalendarDate dateFromShortDateString: value
                              andShortTimeString: nil
                                      inTimeZone: nil];

to

  date = [NSCalendarDate dateFromShortDateString: value
                              andShortTimeString: nil
                                      inTimeZone: [NSTimeZone timeZoneWithName: @"GMT"]];

This should give a consistent result which is not influenced by set setting in SOGoTimeZone.