My server is up to date, must be a bug somewhere still.
EDIT: seems that most of the month old posts showing up are from one particular user at beehaw
EDIT2: and now it appears to be working properly (at least there are no month-old zero-comment posts on the first page.)
If you post to a community that is hosted on a different server, your post is still stored on your home server. The community server is basically just the aggregator. If the community or server goes down the posts are still hosted on their respective home servers.