Default values skew data

In Raymond Chen's brilliantly diverse blog The Old New Thing, he asks:

Why are so many fake LiveJournal blogs written by 29-year-olds?

The answer probably lies in default values for datetime variables: January 1, 1980 is the "year zero" for DOS date/time values, and - lo and behold - anyone born on that date is 29 years old in 2009.