perf: speed up date formatting

bench.js

@@ -50,22 +50,38 @@ const arraySchemaCJS = {
   items: schemaCJS
 }
 
+const dateFormatSchema = {
+  description: 'Date of birth',
+  type: 'string',
+  format: 'date'
+}
+
+const dateFormatSchemaCJS = {
+  description: 'Date of birth',
+  type: 'string',
+  format: 'date'
+}
+
 const obj = {
   firstName: 'Matteo',
   lastName: 'Collina',
   age: 32
 }
 
+const date = new Date()
+
 const multiArray = []
 
 const CJS = require('compile-json-stringify')
 const CJSStringify = CJS(schemaCJS)
 const CJSStringifyArray = CJS(arraySchemaCJS)
+const CJSStringifyDate = CJS(dateFormatSchemaCJS)
 const CJSStringifyString = CJS({ type: 'string' })
 
 const FJS = require('.')
 const stringify = FJS(schema)
 const stringifyArray = FJS(arraySchema)
+const stringifyDate = FJS(dateFormatSchema)
 const stringifyString = FJS({ type: 'string' })
 let str = ''
 
@@ -138,6 +154,18 @@ suite.add('compile-json-stringify obj', function () {
   CJSStringify(obj)
 })
 
+suite.add('JSON stringify date', function () {
+  JSON.stringify(date)
+})
+
+suite.add('fast-json-stringify date format', function () {
+  stringifyDate(date)
+})
+
+suite.add('compile-json-stringify date format', function () {
+  CJSStringifyDate(date)
+})
+
 suite.on('cycle', cycle)
 
 suite.run()

index.js

@@ -285,10 +285,7 @@ function $asDatetime (date, skipQuotes) {
 function $asDate (date, skipQuotes) {
   const quotes = skipQuotes === true ? '' : '"'
   if (date instanceof Date) {
-    const year = new Intl.DateTimeFormat('en', { year: 'numeric' }).format(date)
-    const month = new Intl.DateTimeFormat('en', { month: '2-digit' }).format(date)
-    const day = new Intl.DateTimeFormat('en', { day: '2-digit' }).format(date)
-    return quotes + year + '-' + month + '-' + day + quotes
+    return quotes + new Date(date.getTime() - (date.getTimezoneOffset() * 60000 )).toISOString().slice(0, 10) + quotes
   } else if (date && typeof date.format === 'function') {
     return quotes + date.format('YYYY-MM-DD') + quotes
   } else {