isSameDay returns true even if 2 days are in different days in UTC

[zuanoc]

I am kind of confused about the result of this following code

const startTime = parseISO('2022-10-25T21:43:00.000Z');
const endTime = parseISO('2022-10-26T06:43:00.000Z');

console.log(startTime.toISOString(), getISODay(startTime), getDate(startTime));

console.log(endTime.toISOString(), getISODay(endTime), getDate(endTime));

console.log(isSameDay(startTime, endTime));

it prints:

2022-10-25T21:43:00.000Z 3 26
2022-10-26T06:43:00.000Z 3 26
true

The start time is at2022-10-25T21:43:00.000Z UTC and endTime is at 2022-10-26T06:43:00.000Z UTC. They are different days but isSameDay returns true. Does anyone know if i do anything wrong?
My time zone is Finland, UTC+3

Thank you.

[fturmel]

The start time is at 2022-10-25T21:43:00.000Z UTC and endTime is at 2022-10-26T06:43:00.000Z UTC. They are different days but isSameDay returns true.

They are the same day in your local timezone, which is what date-fns operates on.

// in node.js
process.env.TZ = 'Europe/Helsinki';

import { isSameDay, parseISO } from 'date-fns';

const startTime = parseISO('2022-10-25T21:43:00.000Z');
const endTime = parseISO('2022-10-26T06:43:00.000Z');

console.log(startTime.toString()); // Wed Oct 26 2022 00:43:00 GMT+0300 (Eastern European Summer Time)
console.log(endTime.toString());   // Wed Oct 26 2022 09:43:00 GMT+0300 (Eastern European Summer Time)
console.log(isSameDay(startTime, endTime)); // true

[Semigradsky]

date-fns doesn't work with dates in UTC