def first_day_of(klass_name)
      klass = klass_name.to_s.constantize
      klass.order("created_at ASC").first.try(:created_at) || Time.now
    end
def first_day_of(klass_name)
      klass = klass_name.to_s.constantize
      klass.order("created_at ASC").first.try(:created_at) || Time.now
    end
returns the earliest `created_at` of a given class
 returns `Time.now` if none is available