This is how it is in the knowledge base:

In all calendar systems, there must be one backwards era of infinite length, and one forwards era of infinite length (equivalent to our BC and AD), so that any point in time can be correctly labelled. In some calendar systems these may represent pre-historic times and unknown future times.

It also took me a while to find out how it works with the eras, but it’s quite simple.

“BC” and “AD” cannot be deleted, so these eras are simply renamed. For example, I have named them “First Age” and “ Final Age”. In between, you can then insert your own ages in the desired order and length. In my case (Aeon Timeline 2), this looks like this in the calendar settings:

The eras follow one another in the timeline. Here, for example, is the transition between two eras: