habitica/migrations/20140119_task_creation_completion_dates.js

db.users.find({},{todos:1,dailys:1,rewards:1,habits:1}).forEach(function(user){
  _.each(user.habits.concat(user.dailys).concat(user.todos).concat(user.rewards), function(t){
    t.dateCreated = t.created || new Date;
    delete t.created;
    if (t.type == 'todo' && t.completed) t.dateCompleted = new Date;
  })
  db.users.update({_id:user._id}, {$set:{habits:user.habits,dailys:user.dailys,todos:user.todos,rewards:user.rewards}});
});