# model.rb

def starts_at_day=(day)
    self.starts_at = self.starts_at.sub(/\d\d$/, day)
end

# test

def test_starts_at_day_is_writeable
     s = Story.create :name => "foobar", :starts_at => "1111", :ends_at => "0909"
     # Is date the only part that is changed?
     s.starts_at_day = "22"
     assert s.save
     assert_equal "1122", s.starts_at
end

# error when trying through webinterface

You have a nil object when you didn't expect it!
The error occurred while evaluating nil.sub