Wednesday, February 24, 2016

A Nice Rant About Representing Computer System Time

Here's a nice rant about dealing with time, time zones, daylight savings time, leap seconds, and why keeping things straight is so difficult. This especially applies to embedded systems which might not have a network connection, let alone access to a networked time service.

 The overall video series looks pretty interesting too, although it's more about computers in general than embedded computers specifically.  (Scroll down past the video link to see a list of how time-keeping bugs have caused severe outages and even deaths.)

"Summary: Published on Dec 30, 2013 A web app that works out how many seconds ago something happened. How hard can coding that be? Tom Scott explains how time twists and turns like a twisty-turny thing. It's not to be trifled with! (Embedded from YouTube; <computerphile>)"

Bonus content: here is a rogue's gallery of time goofs that I happen to include in my classroom lectures.  If you know of other high profile outages or worse please submit as comments and I'll update the list as we go. (Note: this is only bugs caused by bad time-keeping, not all software outages):
  • Feb. 1991: Patriot missile failure due to floating point time roundoff; 28 deaths (link)
  • Dec 31, 1999: Y2K
  • Feb. 2007: F-22 raptor computer system crash due to crossing the international date line (link)
  • Feb. 2008: Microsoft Zunes basically bricked by leap year bug (link
  • Mar. 2011:  iPhones spring back instead of springing forward (link)
  • Mar. 2012: Windows Azure leap-year bug takes down G-cloud (link)
  • Jul. 1, 2012: Leap second bug wreaks havoc upon on-line services using NTP (link)
  • Sep. 2013: Deep Impact comet mission ends due to calendar date rollover fault (link)
  • May 2015: Boeing 787 timer rollover bug crashes engine software after 248 days (link)
  • Jul. 1, 2015: Another leap second problem, but not nearly as bad as 2012 (link)
  • Feb. 2016: iPhone prank bricks iPhones if date set back to zero Unix time (link)
  • Feb. 2016: Leap year bug leaves passengers without bags at Dusseldorf airport (link)
  • Jan. 19, 2038 03:15:07 GMT:  Unix time rolls over ("Y2K for Unix")
(There are relatively few listings before 2010.  Don't think for a minute that time suddenly got harder. What got worse was probably more things can be broken and result in headlines due to timekeeping faults.)

Update 11/8/2016:  This is an entertaining and enlightening post about the quirks of time:  Falsehoods programmers believe about time and it has a follow-up of additional falsehoods

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