Why/how does the iOS 1970 bug work?
According to http://www.macrumors.com/2016/02/15/apple-to-fix-january-1-1970-date-bug-ios/:


  Apple has not provided a reason for the bug, but YouTube video maker
  and programmer Tom Scott speculates that setting the date close to
  January 1, 1970, which is 00:00:00 in Unix time, may be resulting in
  an integer underflow -- in this case, a date prior to January 1, 1970.
  
  iOS then handles the underflow by returning the negative integrer to
  the maximum value, which Scott says results in a date that is some 20
  times longer than the universe is expected to last. Scott believes iOS
  may have difficulties handling this large number, resulting in
  affected devices crashing.