It would be better to make it a special month. Nicer for people born on those days than being born on the Unmonth.
Excel would throw a fit, but I propose that numeric date formats handle these unmonthed days as follows:
US-en: __/01/1989, GB-en: 01-__-1989, ISOish: 1989-01-__
That Javascript format: Blessed ___ 01 1989 15:42:03 GMT-0800 (Pacific Standard Time)
like the Giliacs!