The now-defunct space shuttle program was over 30 years old by the time it was decommissioned. It's no surprise that the computer software that ran it had some kinks that people had to work around. One of them was dates.
The computer program does not recognize when the calendar year switches. It would recognize January 1st as day 366 of the previous year. To fix the problem, they would have to basically reboot the shuttle, which would mean a period of time with no navigation updates or control of the vehicle.
This had some real world effects: In 2006, NASA moved a Space Shuttle launch by almost 2 weeks just to give the Space Shuttle more time before New Years. Learn more about the space shuttle program at the source.