Nico uniquely has a rolling comment list with the most recent comments replacing past comments. However, users can filter the comment list by date to view comments from that point in time. That's reflected in requests made to https://nvcomment.nicovideo.jp/v1/threads containing:

"additionals":{"when":1210714200}

We should offer a flag to specify a timestamp (either epoch or %Y-%m-%d %H:%s) and/or to fetch all comments between two timestamps and aggregate them to one file.