The period of time spanning from 19,109 to 20,034 which is enveloped by an inaccessable paradox negation ward. Within that span there exist several parallel timelines which are all sealed away from having any effect on future or past events outside of this period. The Shortest Millennium exists for a single instant at dawn on Primanary 1st, 19,109, at the very beginning of the Four Underdogs period--a thousand years in a metaphorical (metaphysical?) nutshell.
The primary timeline that runs in the main time stream incorporates several elements from the fractured timeline and includes the Four Underdogs defeating (for a time at least) the evil Char Reyarteb and founding the Cerulean Citadel.
See: Countdown to Infinite Wizbits