So after searching through a bunch of these posts regarding some Birthdays in iCal being early or late by one day. Some but not all of mine were early by one day.
I realised that I had been across the dateline with my computer and had not reset or turned off the computer since this. I reset my MacBook Air and Voila! all my Birthdates are showing on the correct day. This should work with both directions of the dateline for one day ahead or one day behind. I guess I need to turn my computer off and on again more often then I thought.
I hope this corrects this issue for everyone else as well.