How do I get the last modified date of a directory in Amazon S3?

So I'm aware that Amazon S3 doesn't really have directories. My question is: does this make it impossible to reliably get the last-modified timestamp of a "directory" in S3?

I know you can get the last-modified date of a file, as in this question.

I say "reliably" because it would be possible to define the latest last-modified timestamp of a file inside a directory as the last-modified timestamp of the directory. But that's not really accurate, since if a file inside a directory gets deleted, it wouldn't register as a change to that directory (indeed the deletion might cause the last-modified date to go backwards in time).

We're using boto to scrape S3.