title: NSTimeInterval to unix timestamp
body: <p>I'm getting  <strong>CMDeviceMotion</strong> objects from <strong>CMMotionManager</strong>. One of the properties of the CMDeviceMotion is timestamp, which is expressed as a NSTimeInterval (double). This allows for "sub millisecond" timestamp precision, according to documentation.</p>
  
  <pre><code>[motionManager startDeviceMotionUpdatesToQueue:motionQueue withHandler:^(CMDeviceMotion *motion, NSError *error) { 
    NSLog(@"Sample: %d Timestamp: %f ",counter,  motion.timestamp);
  }
  </code></pre>
  
  <p>Unfortunately, NSTimeInterval is calculated since last device boot, posing significant challenges to using it in its raw form.</p>
  
  <p>Does anyone have a working code to convert this NSTimeInterval into a Unix like timestamp (UTC timezone)? </p>
  
  <p>Thank you!</p>