999 was a Year of Kahless in the Klingon calendar, at least part of which fell in 2373.

The IKS Rotarran's mission to find the IKS B'Moth took place in this year. (DS9: "Soldiers of the Empire")

According to Ron D. Moore, the peculiar choice of year was a random idea he threw in in case the writers might want to follow up on it later. (AOL chat, 1997). However, no follow-up appears to have happened.