Let's say I want to start downloading messages dated from 2015 on from a Yahoogroup, this specific Yahoogroup has messages that go back to 2007. How do I start my download at 2015, and not include any messages older than that?
Right click on the group name and choose Group Settings-- > Download Settings.
You can choose the message number to start at.
Some experimentation will narrow down the correct start message number.
Cheers,
Wilson.