improve timezone offset performance

DateFormatter.js

@@ -33,8 +33,11 @@ DateFormatter.prototype.format = function format(dateFormat, date) {
 
 function buildFormatter(dateFormat) {
   var tokens = tokenizer.tokenize(dateFormat);
-  var functionString = 'return `' + tokens.map(processToken).join('') + '`';
-
+  var functionString = '';
+  if (tokens.includes('Z') || tokens.includes('ZZ')) {
+    functionString += 'var offset = now.getTimezoneOffset();\n';
+  }
+  functionString += 'return `' + tokens.map(processToken).join('') + '`';
   return Function(
     'now',
     'padZero2',
@@ -50,8 +53,8 @@ function buildFormatter(dateFormat) {
 }
 
 var tokenToReplacement = {
-  ZZ: '${now.getTimezoneOffset() >= 0 ? \'-\':\'+\'}${padZero2(Math.abs(~~(now.getTimezoneOffset() / 60)))}:${padZero2(now.getTimezoneOffset() % 60)}',
-  Z: '${now.getTimezoneOffset() >= 0 ? \'-\':\'+\'}${padZero2(Math.abs(~~(now.getTimezoneOffset() / 60)))}${padZero2(now.getTimezoneOffset() % 60)}',
+  ZZ: '${offset >= 0 ? \'-\':\'+\'}${padZero2(Math.abs(~~(offset / 60)))}:${padZero2(offset % 60)}',
+  Z: '${offset >= 0 ? \'-\':\'+\'}${padZero2(Math.abs(~~(offset / 60)))}${padZero2(offset % 60)}',
   SSS: '${padZero3(now.getMilliseconds())}',
   ss: '${padZero2(now.getSeconds())}',
   s: '${now.getSeconds()}',

benchmark/benchmarkOffset.js

@@ -0,0 +1,35 @@
+/* eslint-disable no-console */
+'use strict';
+
+var Benchmark = require('benchmark');
+var moment = require('moment');
+var dateFormat = require('dateformat');
+var format = require('date-format');
+
+var DateFormatter = require('../DateFormatter');
+var dateFormatter = new DateFormatter();
+
+var DATE_FORMAT = ('ZZ');
+
+const suite = new Benchmark.Suite();
+
+suite
+  .add('moment', () => {
+    moment().format(DATE_FORMAT);
+  })
+  .add('date-format', () => {
+    format(DATE_FORMAT, new Date());
+  })
+  .add('dateformat', () => {
+    dateFormat(new Date(), DATE_FORMAT);
+  })
+  .add('fast-date-format', () => {
+    dateFormatter.format(DATE_FORMAT)
+  })
+  .on('cycle', function(event) {
+    console.log(String(event.target));
+  })
+  .on('complete', function() {
+    console.log('Fastest is ' + this.filter('fastest').map('name'));
+  })
+  .run({ 'async': true });