How to download only newer versions of a file using FTP

[Ryan Golhar]

I thought this was a simply problem but I'm having trouble finding the
solution to this.

There a a couple of files on an FTP site that I want to routinely
download whenever they are updated ie only download newer versions of
the file.  I looked at using ftp, curl, wget, and rsync, but none seem
to have the options necessary to do this.  

Does anyone know how I can do this?  Thanks,

Ryan