Why can't the metadata of a directory include the last modified time?

When using some file systems, such as local file systems, the metadata of directories generally includes the last modified time. However, in the list of entries traversed by opendal, if the entry is a directory, its last modified time is None.
If I want to obtain the last modified time of a directory, how should I do it?

No file or test is named. Start by tracing how OpenDAL builds metadata for directory entries and how local filesystem directory timestamps are mapped; compare that path with the behavior described for local filesystems. Done means supported directory last-modified times are exposed consistently, with coverage for the relevant traversal or metadata behavior.

Written by the indexing model from the issue text.