When this method was implemented
(https://github.com/rails/rails/pull/5016/files), `to_date` is called.
But this behavior was chagned refactoring
(https://github.com/rails/rails/pull/6452/files).
In the first commit, there were not test which asserts `to_date` is called.

I think trying `to_date` is more useful than trying `strftime`, because
we can write `"2015-01-01".to_date`. But first fix comments to match
actual behavior.