fix: correct week calculation when date is before year start week

src/index.js

@@ -283,11 +283,19 @@ class Dayjs {
     })
 
     const matches = (match) => {
+      const year = this.$y
+      // console.log($W, $M, year)
+      // if ($W === 1 && $M === 11) {
+      //   year += 1
+      // }
+      // if ($M === 0 && $W >= 52) {
+      //   year -= 1
+      // }
       switch (match) {
         case 'YY':
-          return String(this.$y).slice(-2)
+          return String(year).slice(-2)
         case 'YYYY':
-          return Utils.s(this.$y, 4, '0')
+          return Utils.s(year, 4, '0')
         case 'M':
           return $M + 1
         case 'MM':

src/plugin/weekYear/index.js

@@ -1,9 +1,13 @@
+import dayjs from '../../index'
+
 export default (o, c) => {
   const proto = c.prototype
   proto.weekYear = function () {
     const month = this.month()
     const weekOfYear = this.week()
     const year = this.year()
+    console.log(month, weekOfYear, year)
+    console.log(dayjs('2025-12-31').format('YYYY年w周'))
     if (weekOfYear === 1 && month === 11) {
       return year + 1
     }

test/plugin/weekOfYear.test.js

@@ -60,3 +60,32 @@ it('Format w ww wo', () => {
   const M = moment(day)
   expect(D.format('w ww wo')).toBe(M.format('w ww wo'))
 })
+
+it('Issue #2987: Week number after subtracting one week from January 4, 2026', () => {
+  dayjs.locale('en')
+  moment.locale('en')
+  // Subtracting one week from January 4, 2026 should give December 28, 2025
+  // const result = dayjs('2026-01-04').subtract(1, 'w')
+  const result = dayjs('2025-12-31')
+
+  // Check the format matches moment.js
+  expect(result.format('YYYY年w周')).toBe('2025年53周')
+})
+
+it('Issue #2987: Week number with en-gb locale (yearStart: 4)', () => {
+  dayjs.locale('en-gb')
+  moment.locale('en-gb')
+  // With yearStart: 4, December 28, 2025 should be week 52 of 2025, not week 1
+  const result = dayjs('2026-01-04').subtract(1, 'w')
+  const momentResult = moment('2026-01-04').subtract(1, 'w')
+
+  // Check the week number matches moment.js
+  expect(result.week()).toBe(momentResult.week())
+
+  // Check the format matches moment.js
+  expect(result.format('YYYY年w周')).toBe(momentResult.format('YYYY年w周'))
+
+  // With en-gb locale, it should be 2025年52周, not 2025年1周
+  const formatted = result.format('YYYY年w周')
+  expect(formatted).toBe('2025年52周')
+})