title: isDaylightSavingTimeForDate causing fatal error
body: <p>fatal error: unexpectedly found nil while unwrapping an Optional value</p>
  
  <p>Routine triggering this error is:</p>
  
  <pre><code>// Calculate Timer Offset
  let tz = NSTimeZone(name: inputTimeZone)
  if tz!.isDaylightSavingTimeForDate(newDate!) {
       inputTimeOffset = inputTimeOffset + inputDayLightSaving
  } 
  </code></pre>
  
  <p>when DayLightSaving changes (newDate!/30th March). Both inputTimeOffset and inputDayLightSaving have a value of 3600.</p>