The data type of SPS-Birthday property is date no year which means all the dates will have a fixed year (which is 2000). This is the default behavior of Microsoft for date no year data type.

By default, when you try to edit a user profile property, SharePoint will not ask you for the year, as shown below:

However, if you are importing user profiles from Active Directory OR you are trying to set the user profile birthday property programmatically (using Power Shell, c# or any other method), you will have to make sure the years of dates must be 2000, in order to make this ShortPoint elements work property.