Web Wiz Forums does not set a default date if no Date of Birth is given by the member.
For the DOB field to be populate for a member they have to select from the DOB drop down a year, month, and day. If any of these are left out the DOB field in the database is not populated.
On this forum out of 15,717 members 12,478 have not entered a DOB and this field is not populated for them. However if a run a search for people with a birthday on 1st Jan 567 are returned.
These 567 people have all selected a DOB in their profile and have just selected the first choice in the drop down for the year, month, and day.
The problem is that because there is no default date being set it's not really a bug as these people are being lazy or not wanting to give their real birthday and select 1st Jan themselves as their DOB.
The problem then is how would you fix this? If you rejected birth days that people enter as the 1st Jan then those people who really do have a birthday on this day would not be able to enter one.