It looks like you encountered one of the FTP server that don't support the MDTM command (used to retrieve a date/time of a single file by
GetFileDateTime method. This command was not originally a part of FTP protocol, and some servers have never added support for it.
As a workaround, you would have to use the
GetList method to retrieve a list of all files in a directory. Then, find the latest file in the list - the returned list is a list of
FtpItem objects with LastWriteTime property set.