date-utils: getWeekDays has incoherent behavior when both locale and startOfWeekProp are set

[loic-simon]

🐛 Bug report

The function getWeekDays exported in @zag-js/date-utils does not respect startOfWeekProp argument in some locales (ex. "fr").

💥 Steps to reproduce

import { CalendarDate } from "@internationalized/date";
import { getWeekDays } from "@zag-js/date-utils";

function test(locale, weekStartDay) {
  const today = new CalendarDate(2024, 8, 21);
  const days = getWeekDays(today, weekStartDay, "UTC", locale);
  return days[0].value.day;
}

console.log(test("fr", 1));

Outputs:

20

Which is a Tuesday (August 20, 2024).

💻 Link to reproduction

CodeSandbox reproduction: https://codesandbox.io/p/devbox/naughty-wood-6qnkw4

🧐 Expected behavior

test("fr", 1) should return 19 (week starts on Monday):

• test("fr", undefined) returns 19 as expected (Monday being the default for "fr" locale);
• test("en", 1) also returns 19 as expected (asking explicitely to start week on Monday);
• but passing the two options returns 20.

🧭 Possible Solution

The bugs comes from getStartOfWeek private function; I believe that if firstDayOfWeek is provided, then the locale should not matter.

🌍 System information

Software	Version(s)
Zag Version	0.65.1
Browser	N/A
Operating System	N/A

📝 Additional information

N/A