There is a claim circulating on Facebook that space shuttles couldn't fly near the end of the year because its onboard computers could not handle a year turnover.

Searching for the claim, I found Y2K-like fears create shuttle scheduling crunch, which says,

The space shuttle's computer software is about 30 years old and does not recognise when the calendar year switches. On 1 January 2007, for example, it will think it is day 366 of 2006 - a problem NASA calls 'year-end rollover'.

To reset the time, the shuttle's main computers would have to be 'reinitialised', which would mean a period without navigation updates or vehicle control, a situation NASA obviously wants to avoid.

As a programmer I know I should always store time as a Unix timestamp or some other T+x time representation, so I don't need to deal with timezones or other stuff. I only need to convert to or from actual date and time when presenting it or when parsing user input.

So I cannot believe NASA made such a mistake in that critical software.

  • NASA does indeed use day and time-of-day. Day is sometimes expressed as Julian day, which is supposed to rollover at the end of the year, but more frequently uses mission day, which is always monotonically increasing.
    – Ben Voigt
    Commented Jan 2, 2015 at 19:12
  • 5
    When I look through a list of Space Shuttle missions, I see none have been in orbit during New Year, and the one that took place latest in a year, was STS-103 which lifted off (after some abandoned attempts) 1999 December 19 and landed December 27 (local Florida time; this was December 20 to December 28 according to UTC). Commented Jan 2, 2015 at 22:50

1 Answer 1


From an archive from NASASpaceflight.com (which is not an official NASA web site), NASA solves YERO problem for shuttle:

By Chris Bergin, 2/19/2007 6:35:00 PM

Shuttle orbiters can now enjoy a happy New Year's Eve on orbit, following the recommendation to implement a YERO (Year End Rollover) solution that was recently designed by NASA engineers.

The recommendation, which was presented in the form of a 45 page presentation to the all-powerful PRCB (Program Requirements Control Board), involves the implementation of a modification to flight software on the orbiters, and new ground procedures.

The above is a synopsis, see the hyperlink above for the "full story".

  • 1
    "The YERO plan may actually remain unused if all of the remaining shuttle missions launch away the small window that is affected." Reading through the list of missions, it would appear they never needed it. I wonder if it was ever implemented.
    – Schwern
    Commented Jan 3, 2015 at 2:32

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