For reference this is the User Profile Properties section of Sharepoint where you can start an import. Ours is configured to interface with Active Directory. We have mapped the SPS-Birthday field to a custom field in Active Directory since Active Directory doesn't natively store birthdays.
Everytime we try to import a birthday from Active Directory into the SPS-Birthday field in Sharepoint it seems to get absolutely nothing. The SPS-Birthday field remains null. We have tried changing the format of the date in Active Directory, and we are still getting a null value.
We even created a new Profile Property, but gave it a string datatype, and it was able to import the data from our custom Active Directory field. I think the issue might have something to do with the DateTime format that is expected in sharepoint for the SPS-Birthday field.