Here is a better way to do that:
Code:
UPDATE tablename SET datefield= ‘2005-01-01′ + interval rand()*883
where ‘2005-01-01′ is the base date and 883 is the number days (maximum) you can go past the base date.
This will fill in each rows datefield with a random date between 2005-01-01 and 2006-06-02.
Hope that helps!