feat: timezoneOffset option to specify output timezone, see #375

src/builtin/filters/date.ts

@@ -1,9 +1,9 @@
-import strftime, { TimezoneDate } from '../../util/strftime'
+import strftime, { createDateFixedToTimezone } from '../../util/strftime'
 import { isString, isNumber } from '../../util/underscore'
 import { FilterImpl } from '../../template/filter/filter-impl'
 
 export function date (this: FilterImpl, v: string | Date, arg: string) {
-  let date = v
+  let date: Date
   if (v === 'now' || v === 'today') {
     date = new Date()
   } else if (isNumber(v)) {
@@ -12,10 +12,12 @@ export function date (this: FilterImpl, v: string | Date, arg: string) {
     if (/^\d+$/.test(v)) {
       date = new Date(+v * 1000)
     } else if (this.context.opts.preserveTimezones) {
-      date = new TimezoneDate(v)
+      date = createDateFixedToTimezone(v)
     } else {
       date = new Date(v)
     }
+  } else {
+    date = v
   }
   return isValidDate(date) ? strftime(date, arg) : v
 }

src/liquid-options.ts

@@ -6,6 +6,7 @@ import { FS } from './fs/fs'
 import * as fs from './fs/node'
 import { defaultOperators, Operators } from './render/operator'
 import { createTrie, Trie } from './util/operator-trie'
+import { timezoneOffset } from './util/strftime'
 
 export interface LiquidOptions {
   /** A directory or an array of directories from where to resolve layout and include templates, and the filename passed to `.renderFile()`. If it's an array, the files are looked up in the order they occur in the array. Defaults to `["."]` */
@@ -24,6 +25,8 @@ export interface LiquidOptions {
   strictVariables?: boolean;
   /** Modifies the behavior of `strictVariables`. If set, a single undefined variable will *not* cause an exception in the context of the `if`/`elsif`/`unless` tag and the `default` filter. Instead, it will evaluate to `false` and `null`, respectively. Irrelevant if `strictVariables` is not set. Defaults to `false`. **/
   lenientIf?: boolean;
+  /** JavaScript timezoneOffset for `date` filter, default to local time. That means if you're in Australia (UTC+10), it'll default to -600 */
+  timezoneOffset?: number;
   /** Strip blank characters (including ` `, `\t`, and `\r`) from the right of tags (`{% %}`) until `\n` (inclusive). Defaults to `false`. */
   trimTagRight?: boolean;
   /** Similar to `trimTagRight`, whereas the `\n` is exclusive. Defaults to `false`. See Whitespace Control for details. */
@@ -108,6 +111,7 @@ export const defaultOptions: NormalizedFullOptions = {
   lenientIf: false,
   globals: {},
   keepOutputType: false,
+  timezoneOffset: timezoneOffset,
   operators: defaultOperators,
   operatorsTrie: createTrie(defaultOperators)
 }

src/util/strftime.ts

@@ -1,5 +1,7 @@
 import { changeCase, padStart, padEnd } from './underscore'
 
+export const timezoneOffset = new Date().getTimezoneOffset()
+const ISO8601_TIMEZONE_PATTERN = /([zZ]|([+-])(\d{2}):(\d{2}))$/
 const rFormat = /%([-_0^#:]+)?(\d+)?([EO])?(.)/
 const monthNames = [
   'January', 'February', 'March', 'April', 'May', 'June', 'July', 'August',
@@ -125,11 +127,10 @@ const formatCodes = {
   y: (d: Date) => d.getFullYear().toString().substring(2, 4),
   Y: (d: Date) => d.getFullYear(),
   z: (d: Date, opts: FormatOptions) => {
-    const offset = d.getTimezoneOffset()
-    const nOffset = Math.abs(offset)
+    const nOffset = Math.abs(timezoneOffset)
     const h = Math.floor(nOffset / 60)
     const m = nOffset % 60
-    return (offset > 0 ? '-' : '+') +
+    return (timezoneOffset > 0 ? '-' : '+') +
       padStart(h, 2, '0') +
       (opts.flags[':'] ? ':' : '') +
       padStart(m, 2, '0')
@@ -140,12 +141,7 @@ const formatCodes = {
 };
 (formatCodes as any).h = formatCodes.b
 
-export default function (inputDate: Date, formatStr: string) {
-  let d = inputDate
-  if (d instanceof TimezoneDate) {
-    d = d.getDisplayDate()
-  }
-
+export default function (d: Date, formatStr: string) {
   let output = ''
   let remaining = formatStr
   let match
@@ -174,24 +170,30 @@ function format (d: Date, match: RegExpExecArray) {
   return padStart(ret, padWidth, padChar)
 }
 
-export class TimezoneDate extends Date {
-  ISO8601_TIMEZONE_PATTERN = /([zZ]|([+-])(\d{2}):(\d{2}))$/;
-
-  inputTimezoneOffset = 0;
-
-  constructor (public dateString: string) {
-    super(dateString)
-    const m = dateString.match(this.ISO8601_TIMEZONE_PATTERN)
-    if (m && m[1] === 'Z') {
-      this.inputTimezoneOffset = this.getTimezoneOffset()
-    } else if (m && m[2] && m[3] && m[4]) {
-      const [, , sign, hours, minutes] = m
-      const delta = (sign === '+' ? 1 : -1) * (parseInt(hours, 10) * 60 + parseInt(minutes, 10))
-      this.inputTimezoneOffset = this.getTimezoneOffset() + delta
-    }
+/**
+ * Create a Date object fixed to it's declared Timezone. Both
+ * - 2021-08-06T02:29:00.000Z and
+ * - 2021-08-06T02:29:00.000+08:00
+ * will always be displayed as
+ * - 2021-08-06 02:29:00
+ * regardless timezoneOffset in JavaScript realm
+ *
+ * The implementation hack:
+ * Instead of calling `.getMonth()`/`.getUTCMonth()` respect to `preserveTimezones`,
+ * we create a different Date to trick strftime, it's both simpler and more performant.
+ * Given that a template is expected to be parsed fewer times than rendered.
+ */
+export function createDateFixedToTimezone (dateString: string) {
+  const m = dateString.match(ISO8601_TIMEZONE_PATTERN)
+  // representing a UTC datetime
+  if (m && m[1] === 'Z') {
+    return new Date(+new Date(dateString) + timezoneOffset * 60000)
   }
-
-  getDisplayDate (): Date {
-    return new Date((+this) + this.inputTimezoneOffset * 60 * 1000)
+  // has a timezone specified
+  if (m && m[2] && m[3] && m[4]) {
+    const [, , sign, hours, minutes] = m
+    const delta = (sign === '+' ? 1 : -1) * (parseInt(hours, 10) * 60 + parseInt(minutes, 10))
+    return new Date(+new Date(dateString) + (timezoneOffset + delta) * 60000)
   }
+  return new Date(dateString)
 }