The pressure is on at NASA to launch the next Shuttle mission on time, because if the launch date slips past December 18, a computer bug could keep the twelve-day mission grounded until January next year.
The Shuttle was never expected to be in orbit as one year gives way to another, so the computers aren't set up to switch to a new "Day One". To the Shuttle, January 1 is just day 366.
"The shuttle computers were never envisioned to fly through a year-end changeover," space shuttle program manager Wayne Hale explained.
In itself, this wouldn't be a problem, but the computers on the ground work differently, and losing synch with mission control would probably be a Bad Thing.
The onboard computer could be reset, but this would mean that the Shuttle would be flying blind, without navigation updates or vehicle control. Although simulations of the date switch have gone well, understandably, NASA would prefer to avoid this scenario in the real world.
Hale says he thinks a launch in December is likely, though, because the weather in Florida tends to be "more benign" in December, lessening the chances of lightning or hurricane induced delays.
Currently the Shuttle is set to launch on December 7, but NASA says it might move the launch date up to December 6. ®