You can downcast to DbxFiles.FileMetadata, which has a clientModified field. (Of course this only works if the thing you got metadata for is in fact a file. A folder will return DbxFiles.FolderMetadata instead, and deleted items will return DbxFiles.DeletedMetadata.)
We love to learn from the educators who use Dropbox. Whether you teach kids, teens, adults or a combination of all three, we want to know what apps and integrations you use with Dropbox to help with teaching. Which of the ones below is your favorite, or most used tool?