diff --git a/CHANGELOG.md b/CHANGELOG.md
index 85e19ed..450309d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,10 @@ Nothing for now.
 
 Compare with [last published version](https://github.com/meduzen/datetime-attribute/compare/1.3.3...main).
 
+### Fixed
+
+- Fix incorrect year for `datetime(date, 'week')` when the week started the previous year. For example, `2021-01-01` is a Friday and its week belongs to 2020 (as [per spec](./README.md#weeknumber). In that case, the output was `2021-53` instead of `2020-53`.
+
 ### Improved
 
 - Improve type definitions where `?` and `undefined` are redundant by removing `undefined`:
diff --git a/src/datetime.js b/src/datetime.js
index 11d43ff..f81d21c 100644
--- a/src/datetime.js
+++ b/src/datetime.js
@@ -34,14 +34,25 @@ export function datetime(date = (new Date()), precision = 'day') {
   // Cut last 4 digits of the year from the others to ease further computation.
   if (year > 9999) {
     [bigYearDigits, year] = (year / 10000).toFixed(4).split('.')
+    year = parseInt(year)
 
-    // Clone date (to leave it untouched) and set a 4-digits year in the clone.
-    date = new Date(date)
+    // Set a 4-digits year.
     date.setFullYear(year)
   }
 
+  /**
+   * If the week started the previous year (see `weekNumber` documentation) and
+   * the requested precision is 'week', we decrement the year by 1.
+   */
+
+  const weekNum = precision == 'week' ? weekNumber(date) : null
+
+  if (weekNum == 53 && date.getMonth() == 0) {
+    year--
+  }
+
   // Local datetime at milliseconds precision (1960-04-27T00:00:00.123)
-  const localMs = p(year.toString(), 4) // zero-pad years, as per spec
+  const localMs = p(year, 4) // zero-pad years, as per spec
     + '-' + p(date.getMonth() + 1) // `+ 1` because 0 is January and 11 is December
     + '-' + p(date.getDate())
     + 'T' + p(date.getHours())
@@ -49,23 +60,23 @@ export function datetime(date = (new Date()), precision = 'day') {
     + ':' + p(date.getSeconds())
     + '.' + p(date.getMilliseconds(), 3)
 
-  const addSignAndYearDigits = shouldAdd => shouldAdd ? sign + bigYearDigits : ''
+  const withSignAndYearDigits = shouldAdd => shouldAdd ? sign + bigYearDigits : ''
 
   /**
    * Extract substring from local date. When `start` is 0, the year is wanted:
    * its sign and missing digits (for years with 5+ digits) are prepended.
    */
 
-  const local = (start, length) => addSignAndYearDigits(!start) + localMs.substr(start, length)
-  const utc = (start, length) => addSignAndYearDigits(!start) + date.toJSON().substr(start, length) + 'Z'
+  const local = (start, length) => withSignAndYearDigits(!start) + localMs.substr(start, length)
+  const utc = (start, length) => withSignAndYearDigits(!start) + date.toJSON().substr(start, length) + 'Z'
 
   const formats = {
     'year': () => local(0, 4),   // 1960
     'month': () => local(0, 7),  // 1960-04
     'day': () => local(0, 10),   // 1960-04-27
 
-    'week': () => local(0, 5) + 'W' + p(weekNumber(date)), // 1960-W17
-    'yearless': () => local(5, 5),      // 04-27
+    'week': () => local(0, 5) + 'W' + p(weekNum), // 1960-W17
+    'yearless': () => local(5, 5), // 04-27
 
     'time': () => local(11, 5),   // 00:00
     'second': () => local(11, 8), // 00:00:00
diff --git a/src/datetime.test.js b/src/datetime.test.js
index 325051e..f9e5c9d 100644
--- a/src/datetime.test.js
+++ b/src/datetime.test.js
@@ -1,6 +1,7 @@
 import { describe, expect, test } from 'vitest'
 
 import { datetime, datetimeTz, tzOffset, utc } from './index.js'
+import { getNormalizeDay } from './utils/date.js'
 
 const togoIndependanceDay = new Date(1960, 3, 27)
 const date = togoIndependanceDay // alias for the sake of brevity
@@ -87,10 +88,16 @@ describe('datetime', () => {
       expect(datetime(december31th2020, 'week')).toBe('2020-W53')
     })
 
-    test.todo('week on 2021-01-01 is 2020-W53', () => {
+    // 1st day of the year is after Thurdsay
+    test('week on 2021-01-01 is 2020-W53', () => {
+      expect(getNormalizeDay(january1st2021)).toBeGreaterThan(4)
       expect(datetime(january1st2021, 'week')).toBe('2020-W53')
     })
 
+    test('week on 2021-01-08 is 2021-W01', () => {
+      expect(datetime(new Date(2021, 0, 8), 'week')).toBe('2021-W01')
+    })
+
     test('week on 2021-01-11 is 2021-W02', () => {
       expect(datetime(january11th, 'week')).toBe('2021-W02')
     })
@@ -99,6 +106,13 @@ describe('datetime', () => {
       expect(datetime(january19th, 'week')).toBe('2021-W03')
     })
 
+    // 1st day of the month is after Thurdsay
+    test('week on 2021-03-01 is 2021-W17', () => {
+      const march1st2021 = new Date(2021, 4, 1)
+      expect(getNormalizeDay(march1st2021)).toBeGreaterThan(4)
+      expect(datetime(march1st2021, 'week')).toBe('2021-W17')
+    })
+
     test('week on 2021-12-31 is 2021-W52', () => {
       expect(datetime(december31th2021, 'week')).toBe('2021-W52')
     })
diff --git a/src/utils/date.js b/src/utils/date.js
index 2ab2757..1ca8a51 100644
--- a/src/utils/date.js
+++ b/src/utils/date.js
@@ -35,7 +35,7 @@ export function daysBetween(date, furtherDate) {
  * @returns {number}
  */
 export function weekNumber(date) {
-  const dayIndex = date.getDay() || 7 // normalize index because Sunday == 0
+  const dayIndex = getNormalizeDay(date) // normalize index because Sunday == 0
 
   const sameWeekThursday = new Date(date)
   sameWeekThursday.setDate(date.getDate() + 4 - dayIndex)
@@ -45,3 +45,11 @@ export function weekNumber(date) {
 
   return Math.ceil(daysDifference / 7)
 }
+
+/**
+ * Get the day index of the week, except Sunday is 7 instead of 0.
+ *
+ * @param {Date} date
+ * @returns {number}
+ */
+export const getNormalizeDay = date => date.getDay() || 7