What is the proper way to get "server_modified" date from DBFILESMetadata?

You can access the server_modified property on a DBFILESFileMetadata (its not available on DBFILESMetadata itself) like this:

[[client.filesRoutes getMetadata:path] setResponseBlock:^(DBFILESMetadata * _Nullable result, DBFILESGetMetadataError * _Nullable routeError, DBRequestError * _Nullable networkError) {
    if (result) {
        if ([result isKindOfClass:[DBFILESFileMetadata class]]) {
            DBFILESFileMetadata *fileMetadata = (DBFILESFileMetadata *)result;
            NSLog(@"This is a file at path: %@ with server modified time: %@\n", fileMetadata.pathDisplay, fileMetadata.serverModified);
        } else {
            NSLog(@"Not a file.");  // folder (or deleted, if setting includeDeleted:YES)
        }
    } else {
        // error handling
    }
}];