Do you have, or can you install,
lftp? It is a scriptable FTP client that can use several protocols, including SFTP.
One of
lftp's commands is
cls which could be used to make a listing of filename and date which could then be processed locally to identify files to be deleted. It won't be pretty. The man page
can be found here so that you can read it and decide whether you want to go this route. (The man page doesn't actually give any information about 'cls' unfortunately)
Andrew