From 71bed155edf32bff24379930ac684fc783538d8f Mon Sep 17 00:00:00 2001
From: hansdaniels <63750717+hansdaniels@users.noreply.github.com>
Date: Thu, 28 Jan 2021 06:46:42 +0100
Subject: [PATCH] fix: timezone plugin DST error (#1352)

---
 src/plugin/timezone/index.js | 10 +++++-----
 test/plugin/timezone.test.js |  2 ++
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/plugin/timezone/index.js b/src/plugin/timezone/index.js
index 97f24060b..c8f8a4675 100644
--- a/src/plugin/timezone/index.js
+++ b/src/plugin/timezone/index.js
@@ -36,8 +36,6 @@ const getDateTimeFormat = (timezone, options = {}) => {
 export default (o, c, d) => {
   let defaultTimezone
 
-  const localUtcOffset = d().utcOffset()
-
   const makeFormatParts = (timestamp, timezone, options = {}) => {
     const date = new Date(timestamp)
     const dtf = getDateTimeFormat(timezone, options)
@@ -96,9 +94,11 @@ export default (o, c, d) => {
 
   proto.tz = function (timezone = defaultTimezone, keepLocalTime) {
     const oldOffset = this.utcOffset()
-    const target = this.toDate().toLocaleString('en-US', { timeZone: timezone })
-    const diff = Math.round((this.toDate() - new Date(target)) / 1000 / 60)
-    let ins = d(target).$set(MS, this.$ms).utcOffset(localUtcOffset - diff, true)
+    const date = this.toDate()
+    const target = date.toLocaleString('en-US', { timeZone: timezone })
+    const diff = Math.round((date - new Date(target)) / 1000 / 60)
+    let ins = d(target).$set(MS, this.$ms)
+      .utcOffset((-Math.round(date.getTimezoneOffset() / 15) * 15) - diff, true)
     if (keepLocalTime) {
       const newOffset = ins.utcOffset()
       ins = ins.add(oldOffset - newOffset, MIN)
diff --git a/test/plugin/timezone.test.js b/test/plugin/timezone.test.js
index 4feb7bf3d..dfcebf47e 100644
--- a/test/plugin/timezone.test.js
+++ b/test/plugin/timezone.test.js
@@ -101,7 +101,9 @@ describe('Convert', () => {
       const jun = _('2014-06-01T12:00:00Z')
       const dec = _('2014-12-01T12:00:00Z')
       expect(jun.tz('America/Los_Angeles').format('ha')).toBe('5am')
+      expect(jun.tz('America/Los_Angeles').utcOffset()).toBe(-7 * 60)
       expect(dec.tz('America/Los_Angeles').format('ha')).toBe('4am')
+      expect(dec.tz('America/Los_Angeles').utcOffset()).toBe(-8 * 60)
       expect(jun.tz(NY).format('ha')).toBe('8am')
       expect(dec.tz(NY).format('ha')).toBe('7am')
       expect(jun.tz(TOKYO).format('ha')).toBe('9pm')