TRy this,
SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss");
formatter.setTimeZone(TimeZone.getTimeZone("GMT"));
Date requestDate = formatter.parse(serverDateString);
Calendar requestDateCal = formatter.getCalendar();
requestDateCal.setTime(requestDate);
Calendar currentDateCal = Calendar.getInstance();
currentDateCal.setTimeZone(TimeZone.getTimeZone("GMT"));
calculateTimeDiff(currentDateCal.getTimeInMillis(), requestDateCal.getTimeInMillis()); //finds diff between two timestamps