title: Ignoring time-zone changes after saving NSDate
body: <p>My apps are time-zone agnostic: I want the dates and times saved and displayed in a manner that ignores changes in the user's time zone.  Once I've recorded the local date/time of some user action in my app (typically by saving timeIntervalSince1970 in an sqlite DB), I want to show the same date/time string regardless if the user's time zone has changed or not, and regardless of the user's various time zone settings in his device.</p>
  
  <p>For example, if a user's action occurs at 1:15 pm local time in San Francisco, and then he opens my app a few days later in New York, I want that prior action to still appear as 1:15 pm (not 4:15 pm), and even if he's manually set some other time zone in his device (e.g., Chicago).  A second action in New York at 9:00 pm there, should forevermore display as 9:00 pm, even when back in California.</p>
  
  <p>I could achieve my goal by saving every date/time as a string (based on the user's local time zone), and then never use the date methods again -- but then I couldn't do date/time arithmetic (e.g., sorting).</p>
  
  <p>How can I be time-zone agnostic when saving and retrieving dates/times?</p>
  
  <p>(Apple's Calendar app seems to behave this way when Time Zone Override is on, but only if I manually set the event time.)</p>