From 990012ba0f1032a68761fe176e80bc760346d200 Mon Sep 17 00:00:00 2001
From: iamkun <kunhello@outlook.com>
Date: Tue, 27 Oct 2020 15:18:45 +0800
Subject: [PATCH] fix: fix utc plugin diff bug in DST (#1171)

fix #1097, fix #1021
---
 src/plugin/utc/index.js | 3 +++
 test/timezone.test.js   | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/src/plugin/utc/index.js b/src/plugin/utc/index.js
index 722ae552b..5fb049793 100644
--- a/src/plugin/utc/index.js
+++ b/src/plugin/utc/index.js
@@ -112,6 +112,9 @@ export default (option, Dayjs, dayjs) => {
   }
   const oldDiff = proto.diff
   proto.diff = function (input, units, float) {
+    if (this.$u === input.$u) {
+      return oldDiff.call(this, input, units, float)
+    }
     const localThis = this.local()
     const localInput = dayjs(input).local()
     return oldDiff.call(localThis, localInput, units, float)
diff --git a/test/timezone.test.js b/test/timezone.test.js
index 9c0926f96..13110166d 100644
--- a/test/timezone.test.js
+++ b/test/timezone.test.js
@@ -62,3 +62,11 @@ it('UTC and utcOffset', () => {
   expect(moment.utc(test2).utcOffset(-60).format())
     .toBe(dayjs.utc(test2).utcOffset(-60).format())
 })
+
+it('UTC diff in DST', () => {
+  // DST till 2020-10-25
+  const day1 = dayjs.utc('20201023') // in DST
+  const day2 = dayjs.utc('20201026')
+  expect(day1.diff(day2, 'd'))
+    .toBe(-3)
+})