title: Adding days/hours to NSDate with Swift
body: <p>I know it is a really basic question, and I am aware I might not be finding the answer because I am not asking the right question however how can I change the NSDate value ?  </p>
  
  <p>For Example I have a <code>date</code> property which is set to the date of a <code>datePicker</code> and I want to create a new property which is the day before the <code>date</code> property and another which is an hour before. </p>