I had searched here for a solution and did not find it, but tried again and found it!
https://www.mobileread.com/forums/sho...80#post1736980
The easiest way to automatically get rid of yesterday's feed is to change your recipe to include:
timefmt = ''
(two single quotes) which stops Calibre putting the date in the title. When the feed gets received each day the Kindle 3 automatically archives yesterday's to "Periodicals: Back Issues". Seven issues are retained in this folder before the oldest gets deleted.
I tried this right away - and it works!