Some examples showing how to work with timezones in Apps Script.

Timezones.gs

// Setup: Spreadsheet in Pacific time, Script in Eastern time, Calendar in Mountain time.
// Sample spreadsheet: https://docs.google.com/spreadsheets/d/1neePK1YPKMKnVwI8dld0HSJjI1kSFPpcY8g7kG8xG4U/edit#gid=0
// A2 = 3/1/2016 9:00 AM, B2 = 3/1/2016 10:00 AM

/**
 * The dates and times in the spreadsheet refer to exact moments in time, and the clock
 * on the wall may show a different time.
 */
function copyMoment() {
  var ss = SpreadsheetApp.getActive();
  var sheet = ss.getSheets()[0];
  var startDate = sheet.getRange('A2').getValue();
  var endDate = sheet.getRange('B2').getValue();
  
  var calendar = CalendarApp.getCalendarsByName('Mountain Time')[0];
  calendar.createEvent('Exact Copy', startDate, endDate);
  // Result: Calendar event at 10:00 AM Mountain Time
}

/**
 * The dates and times in the spreadsheet refer to relative moments in time,
 * and the clock on the wall should match.
 */
function copyRelative() {
  var ss = SpreadsheetApp.getActive();
  var sheet = ss.getSheets()[0];
  var startDate = sheet.getRange('A2').getValue();
  var endDate = sheet.getRange('B2').getValue();
 
  var calendar = CalendarApp.getCalendarsByName('Mountain Time')[0];
  
  // Change the timezone of the start and end date to match the calendar.
  startDate = changeTimezone(startDate, ss.getSpreadsheetTimeZone(), calendar.getTimeZone());
  endDate = changeTimezone(endDate, ss.getSpreadsheetTimeZone(), calendar.getTimeZone());
  
  calendar.createEvent('Relative Copy', startDate, endDate);
  // Result: Calendar event at 9:00 AM Mountain time
}

/**
 * Get the same relative date and time in a different timezone.
 */
function changeTimezone(date, fromTimezone, toTimezone) {
  var dateString = Utilities.formatDate(date, fromTimezone, 'yyyy/MM/dd HH:mm:ss');
  var offset = Utilities.formatDate(date, toTimezone, 'Z');
  return new Date(dateString + offset);
}