bug with addDays()?

[77it]

When testing addDays() I found a strange behaviour: date 1917/04/01 + 1 day is not equal to 1917/04/02, that is wrong.
Instead, the date 1917/03/31 + 2 days is equal to 1917/04/02, which is correct.

Follows the code

import * as datefns from 'https://esm.run/date-fns@3.6.0';

const date_20170401 = new Date(1917, 3 /* Apr */, 1);
const date_20170402 = new Date(1917, 3 /* Apr */, 2);
const date_20170401_plus1 = datefns.addDays(date_20170401, 1);

if (date_20170401_plus1.getTime() !== date_20170402.getTime()) {
  console.log(`date_20170331_plus1_plus1: ${date_20170401_plus1}`);
  console.log(`date_20170402: ${date_20170402}`);
  console.log("test #1 failed");
}

const date_20170331 = new Date(1917, 2 /* Mar */, 31);
const date_20170331_plus2 = datefns.addDays(date_20170331, 2);

if (date_20170331_plus2.getTime() !== date_20170402.getTime()) {
  console.log(`date_20170331_plus2: ${date_20170331_plus2}`);
  console.log(`date_20170402: ${date_20170402}`);
  console.log("test #2 failed");
}

[fturmel]

I can't reproduce the problem. Which version of date-fns is this? Can you share your IANA time zone and which JS runtime you are using? What are the console.log() outputs?

This might overlap a daylight saving transition, but I can't think of a reason why the dates wouldn't match up.

[77it]

I'm in Italy, zone Europe/Rome

I tried with Nodejs version v21.6.2 and Deno version deno 1.41.0, same result.

Test run with Nodejs node --experimental-network-imports test.mjs

date_20170331_plus1_plus1: Mon Apr 02 1917 01:00:00 GMT+0200 (Ora legale dell’Europa centrale)
date_20170402: Mon Apr 02 1917 00:00:00 GMT+0200 (Ora legale dell’Europa centrale)
test #1 failed

Test run with Deno deno run test.mjs

date_20170331_plus1_plus1: Mon Apr 02 1917 01:00:00 GMT+0200 (Ora legale dell’Europa centrale)
date_20170402: Mon Apr 02 1917 00:00:00 GMT+0200 (Ora legale dell’Europa centrale)
test #1 failed

Same output running the code online with Chrome with https://playcode.io/1834998

For my needs I use the following code to add days; it ignores the time part and works with any day I've tried:

new Date(date.getFullYear(), date.getMonth(), date.getDate() + amount)

PS
I updated the code posted in the issue to add the import statement.

[fturmel]

Thanks for the additional details. Here's what's happening:

new Date(1917, 3, 1) is actually Sun Apr 01 1917 01:00:00 GMT+0200 (Central European Summer Time) because it is exactly during a daylight saving transition and midnight technically doesn't exist.

new Date(1917, 3, 2) is Mon Apr 02 1917 00:00:00 GMT+0200 (Central European Summer Time) as expected.

So if you add a day to April 1st at 1AM, you get April 2nd at 1AM, not midnight.

---

If you don't want to take into account local time zone and DST, you should use UTC instead, either through the UTCDate package :

// process.env.TZ = 'Europe/Rome';

import { UTCDate } from '@date-fns/utc';
import { addDays } from 'date-fns';

const april1 = new UTCDate(1917, 3, 1);
const april2 = new UTCDate(1917, 3, 2);
const april1Plus1Day = addDays(april1, 1);

console.log(april1.toISOString());
console.log(april2.toISOString());
console.log(april1Plus1Day.toISOString());

// 1917-04-01T00:00:00.000Z
// 1917-04-02T00:00:00.000Z
// 1917-04-02T00:00:00.000Z

...or directly with the native JS Date for such a simple operation:

// process.env.TZ = 'Europe/Rome';

const april1 = new Date(Date.UTC(1917, 3, 1));
const april2 = new Date(Date.UTC(1917, 3, 2));
const april1Plus1Day = new Date(april1);
april1Plus1Day.setUTCDate(april1.getUTCDate() + 1);

console.log(april1.toISOString());
console.log(april2.toISOString());
console.log(april1Plus1Day.toISOString());

// 1917-04-01T00:00:00.000Z
// 1917-04-02T00:00:00.000Z
// 1917-04-02T00:00:00.000Z