Originally Posted by Zunaid
A little erudition is of the order - many computer systems computes time deltas from a January 1, 1970 start date. For example Un*x epoch time %s displays time in seconds since 1970-01-01. Rather than enforcing users to chose a particular date or allow for no dates, lazy programmers will often pre-fill a date field with the deafult start date.
Hence we have so many 40 year olds celebrating their Birthday tomorrow.
In addition to the above:
the reason they got set to Jan 1, 1970 is because they registered in the old forum; and never came back when forum software was upgraded. In the old forum, DOB wasn't required to register. During user account migration all users who didn't have dob, got set to default dob date.
There, you have the whole story now