title: Subtracting hours from datetime format
body: <p>I have a date-time field which returns the value as: 2013-02-01T10:00:00Z
  I want to substract 4hrs from the time (I want the time as GMT-4)
  So the output should be 2013-02-01T06:00:00Z</p>
  
  <p>Can somebody help me how to achieve this?</p>
  
  <p>Thanks,
  Nitika</p>