Deep Impact's chief mission scientist Mike A'Hearn said, "Basically, it was a Y2K problem, where some software didn't roll over the calendar date correctly." The fault-protection software misread any dates after 2013-08-11, and the misreads triggered an endless series of computer reboots. The failure was in the fault-protection software -- i.e., if they hadn't tried to prevent failure, Deep Impact would still be working. What timescale wraps around on 2013-08-11? Perhaps it was something mission-specific, such as the number of deciseconds since the mission began, or the number of milliseconds since the previous reboot, and this was bowdlerized into "calendar date" for the general public. My source: Vargano D. NASA Declares End to Deep Impact Comet Mission. National Geographic 2013-09-20 http://news.nationalgeographic.com/news/2013/09/130920-deep-impact-ends-come...