Testing for a particular date works locally, but not on GitHub

The task is to check if a certain date is a statutory holiday, by comparing a date against a list of known statutory holiday dates.

Here is the JSON list of stat holidays:

{
    "provinces": [
        {
            "name": "Ontario",
            "statutory": [
                {
                    "name": "Christmas",
                    "date": "2025-12-25"
                },
            ]
        ...

Here is the test:

describe("statHoliday", () => {
  test("should identify statutory holiday", () => {
    const statDay = new Date("2025-12-25T10:00:00.000");
    expect(isStatHoliday(statDay)).toBe(true);
  });
});

Here is the function to test the date:

function isStatHoliday(date) {
  const dateObject = new Date(date);
  const dateString = dateObject.toLocaleDateString();

  // "theProvince" here is set to Ontario 
  return !!theProvince.statutory.find(holiday => holiday.date === dateString);
}

When testing locally, this works fine. When I upload to GitHub, and the tests are run there, it fails:

Expected: true
Received: false

I assume this has something to do with time zones, specifically the routine toLocaleDateString(), but I'm not sure how to correct this. Locally, I am GMT-4, and I assume the GitHub server is in UTC?

This issue is likely caused by .toLocaleDateString() producing different results depending on the machine's locale settings.

The format in your JSON is "YYYY-MM-DD", which matches the en-CA (Canadian English) locale. If you want to keep using .toLocaleDateString(), you can make the output consistent across machines by explicitly specifying the locale:

const dateString = dateObject.toLocaleDateString("en-CA")

I tested this myself, and it produced the same result as in your json regardless of my system's locale.