I just checked cache_builders/post_island.php
I didn't do a deep dive, but there is this peace of code:
$one_month = $html->get_date() - (86400 *30);
If you would replace that with:
$one_month = $html->get_date() - (86400 *1000);
That would make it a thousand days instead of 30.
As said, untested.