According to RFC 2616, it's advised to use the same date string that the server sent with the last response when using if-modified-since. http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25

This can probably be handled without causing too much breakage for the FTP fetcher by implementing fileinfo as a class and letting it handle giving out dates in the desired format.