The Fracture

The event that was triggered either by the R.S. Honshu's journey into the distant past in No Time Like the Future, her interactions with the past as detailed in Where the Stars Fall, or her return to the present in The Yesterday Dilemma. The exact cause for the mixing of two separate timelines is unclear to science.

It is this event, which so profoundly affects large swathes of known space, that is the marker for the division between C.E. (common era) and A.F. (after fracture era).