option to only include posts after a certain date in feeds

• I have searched the issues (including closed ones) and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.
• I am willing to lend a hand to help implement this feature.

Feature Request

When migrating a blog to pelican from some other system it would be nice if I could say only include posts after a certain date in the atom feeds. That way existing subscribers won't get flooded with old posts because the new <entry>s have new <id> values. It would mean that new subscribers won't get a few of the old posts in their feed reader but that's a price I'm willing to pay.

You can simulate this by setting the FEED_MAX_ITEMS = 0 and increasing it every time you post but it's a little too manual. FEED_MINIMUM_DATE=2026-08-18 or some such would be really nice.