Board Thread:General Discussion/@comment-29342734-20161217165417/@comment-18172179-20161217221741

Mentions of characters and places are completely optional; meaning that it falls on the editor/author's judgement to include or not the mentioned subjects. Personally, I don't include them unless the character or location was or will be involved later in other issues of the storyline. And sometimes not even then, because I often forget about it.

It is not uncommon to list eras in time. It is often used in stories involving time travel, which were much more common in the Golden and Silver Ages than they are nowadays.