what is the ' forgotten time '?

In this movie, they keep on referring 1991-1997 as the " forgotten time ". But what does that really mean? Now keep in mind, I don't live in Hong Kong so I don't know a lot about its history. Do they mean that the events in this film are never mentioned again in the rest of the series or do they simply mean these are the " dark ages " for the characters? Anyone know?