From 37dbef2b4c26823838e1abb3131a1199aad31017 Mon Sep 17 00:00:00 2001
From: Adam Hellberg <sharparam@sharparam.com>
Date: Tue, 7 Feb 2023 10:18:56 +0100
Subject: [PATCH 1/2] Add missing padding to 'YYYY' format

Adds padding to the year component when formatted with `'YYYY'`, similar to how padding is handled for '`MM`' and '`DD`'.

Fixes #2230
---
 src/index.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/index.js b/src/index.js
index 19c409078..ba74b8b66 100644
--- a/src/index.js
+++ b/src/index.js
@@ -280,7 +280,7 @@ class Dayjs {
 
     const matches = {
       YY: String(this.$y).slice(-2),
-      YYYY: this.$y,
+      YYYY: Utils.s(this.$y, 4, '0'),
       M: $M + 1,
       MM: Utils.s($M + 1, 2, '0'),
       MMM: getShort(locale.monthsShort, $M, months, 3),

From ea97f4cfed6edff8d1985fa5768d63975118ee17 Mon Sep 17 00:00:00 2001
From: Adam Hellberg <sharparam@sharparam.com>
Date: Sat, 11 Feb 2023 14:38:57 +0100
Subject: [PATCH 2/2] Add some tests for 'YYYY' format

---
 test/display.test.js | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/test/display.test.js b/test/display.test.js
index 7eb48fe82..86a978dfc 100644
--- a/test/display.test.js
+++ b/test/display.test.js
@@ -260,3 +260,15 @@ it('As JSON -> toJSON', () => {
 it('As ISO 8601 String -> toISOString e.g. 2013-02-04T22:44:30.652Z', () => {
   expect(dayjs().toISOString()).toBe(moment().toISOString())
 })
+
+it('Year 1 formatted with YYYY should pad with zeroes', () => {
+  const date = new Date(1, 0, 1)
+  date.setUTCFullYear(1) // Required because 0-99 are parsed as 19xx in JS: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date#year
+  expect(dayjs(date).format('YYYY')).toBe('0001')
+})
+
+it('Year 1 formatting matches moment format', () => {
+  const date = new Date(1, 0, 1)
+  date.setUTCFullYear(1)
+  expect(dayjs(date).format('YYYY')).toBe(moment(date).format('YYYY'))
+})