From a9a2db8edebf0342a44c66f5fd1b6c7cfd34c076 Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Tue, 23 Jun 2026 23:34:56 +0100
Subject: [PATCH] fix(duration): apply the leading sign when parsing a negative
 ISO duration

The ISO-8601 duration parser called d.slice(2) and never read the
sign captured in d[1], so a leading-minus string parsed as positive.
toISOString() correctly emits -P..., so the round-trip
duration(d.toISOString()) flipped the sign.

Derive a sign multiplier from d[1] === '-' and apply it to each
parsed component. Scope is the leading-sign case only; per-component
signs are a separate serializer limitation.

Refs #1516
---
 src/plugin/duration/index.js | 4 +++-
 test/plugin/duration.test.js | 7 +++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/plugin/duration/index.js b/src/plugin/duration/index.js
index 5aaacee21..d1f09c90c 100644
--- a/src/plugin/duration/index.js
+++ b/src/plugin/duration/index.js
@@ -84,7 +84,9 @@ class Duration {
       const d = input.match(DURATION_REGEX_PARSE)
       if (d) {
         const properties = d.slice(2)
-        const numberD = properties.map(value => (value != null ? Number(value) : 0));
+        const signMultiplier = d[1] === '-' ? -1 : 1
+        const numberD = properties.map(value =>
+          (value != null ? Number(value) * signMultiplier : 0));
         [
           this.$d.years,
           this.$d.months,
diff --git a/test/plugin/duration.test.js b/test/plugin/duration.test.js
index f9a4c7cf4..e9ce74c16 100644
--- a/test/plugin/duration.test.js
+++ b/test/plugin/duration.test.js
@@ -108,6 +108,13 @@ describe('Parse ISO string', () => {
   it('Invalid ISO string', () => {
     expect(dayjs.duration('Invalid').toISOString()).toBe('P0D')
   })
+  it('Negative ISO string round-trips', () => {
+    expect(dayjs.duration('-P1Y').asYears()).toBe(-1)
+    expect(dayjs.duration('-P1DT2H').asHours()).toBe(-26)
+    expect(dayjs.duration('-PT59S').toISOString()).toBe('-PT59S')
+    const d = dayjs.duration(-1, 'years')
+    expect(dayjs.duration(d.toISOString()).asMilliseconds()).toBe(d.asMilliseconds())
+  })
 })
 
 it('Is duration', () => {