BDAY representation for unknown years

Hello!

What is the intended use of the BDAY field to
store the birthday information where day and
month are known but the year is unknown?

This seems to be quite a common problem if you
create vcards for persons you know (instead
of getting the the vcard from them).

I've read "3.1.5 BDAY Type Definition" of rfc2426
and found no suggestion how to use that field
in case of an unknown year.

The problem arises for the KDE addressbook
implementation [1][2] which uses vcard to store the
information. The problem arises when you want
the system to warn you if a birthday of a
contact gets close.

Users started to use a year 9999 [1] to indicate
an unknown year but this is very likely to
cause troubles when such vcards are copied
onto other devices (mobile phones, pdas, ...).

Regards,
Andreas Schallenberg

References:
[1] http://bugs.kde.org/show_bug.cgi?id=95522
[2] http://bugs.kde.org/show_bug.cgi?id=110049