diff --git a/CHANGELOG.md b/CHANGELOG.md
index 450309d..dfbe93d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,7 +10,7 @@ Compare with [last published version](https://github.com/meduzen/datetime-attrib
 
 ### 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`.
+- 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
 
diff --git a/src/datetime-class.js b/src/datetime-class.js
index f9bee65..7394f42 100644
--- a/src/datetime-class.js
+++ b/src/datetime-class.js
@@ -1,8 +1,16 @@
 import { MILLISECONDS_PER_WEEK } from './utils/const.js'
 import { datetime, weekNumber } from './index.js'
+import { getNormalizeDay } from './utils/date.js'
 
 export class DateTime extends Date {
 
+  /**
+   * Get the day index of the week, except Sunday is 7 instead of 0.
+   *
+   * @returns {number}
+   */
+  getNormalisedDay = () => getNormalizeDay(this)
+
   /**
    * Returns the week of the year (`1`–`53`) of the specified date according to
    * local time, as defined by the WHATWG and the ISO-8601 specs:
diff --git a/src/datetime-class.test.js b/src/datetime-class.test.js
index c5c8748..2307aa8 100644
--- a/src/datetime-class.test.js
+++ b/src/datetime-class.test.js
@@ -45,6 +45,8 @@ describe('DateTime class', () => {
     })
   })
 
+  describe.todo('.getNormalisedDay()', () => {})
+
   describe('.getWeek()', () => {
 
     test('returns the week number', () => {
diff --git a/src/datetime.test.js b/src/datetime.test.js
index f9e5c9d..8e33489 100644
--- a/src/datetime.test.js
+++ b/src/datetime.test.js
@@ -106,7 +106,7 @@ describe('datetime', () => {
       expect(datetime(january19th, 'week')).toBe('2021-W03')
     })
 
-    // 1st day of the month is after Thurdsay
+    // 1st day of the month is after Thurdsay, but it’s not in January
     test('week on 2021-03-01 is 2021-W17', () => {
       const march1st2021 = new Date(2021, 4, 1)
       expect(getNormalizeDay(march1st2021)).toBeGreaterThan(4)
diff --git a/src/utils/date.js b/src/utils/date.js
index 1ca8a51..ec38e03 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 = getNormalizeDay(date) // normalize index because Sunday == 0
+  const dayIndex = getNormalizeDay(date)
 
   const sameWeekThursday = new Date(date)
   sameWeekThursday.setDate(date.getDate() + 4 - dayIndex)
@@ -47,7 +47,7 @@ export function weekNumber(date) {
 }
 
 /**
- * Get the day index of the week, except Sunday is 7 instead of 0.
+ * Get the day index of the week, but Sunday is now 7 instead of 0.
  *
  * @param {Date} date
  * @returns {number}