Starting with v0.89r, 1999.io maintains a monthly archive page in addition to the home page and story pages.
Here's the archive page for May for this blog.
Part of the change that makes this possible is that the server now returns all the messages of the current month to the editor when it starts up. Without the posts resident on the editing machine there would be no way to build the archive page.
There was a server update too, v0.94s.
Click on the body of a post to edit it.
Click the Update button.
The current monthly archive page will be rebuilt.
It only works for the current month. If you rebuild a post in a previous month it will not build the archive page for that month.