Adding and removing one day using addDays doesn't return original date

[punksta]

import fAddDays from 'date-fns/fp/addDays'

const date = new Date("2060-03-27T01:00:00.000Z");

const addAndRemove =  fAddDays(-1)(
      fAddDays(1)(date)
)

console.log(date)
console.log(addAndRemove)

[punksta]

It adds extra hours for some dates.
More examples: [ new Date("2068-10-28T01:00:00.000Z"), new Date("2005-03-26T01:00:00.000Z") ]

[fturmel]

@punksta Looks like a bug to me as well. It seems to happen when the add operation lands on the exact day and hour of a daylight saving transition. I can reproduce it with 2060-03-27T01:00:00.000Z and 2005-03-26T01:00:00.000Z, but not with 2068-10-28T01:00:00.000Z though.

@tan75 There are DST unit tests for the function, maybe they are not covering this edge case? Does this have implications for other add/sub functions in the library?

Here's a nodejs version that should be reproducible for anyone:

import { addDays } from 'date-fns';

process.env.TZ = 'CET';

// bug
const date1 = new Date(2060, 2, 27, 2);
const res1 = addDays(date1, 1);
console.log(date1.toString()); // Sat Mar 27 2060 02:00:00 GMT+0100 (GMT+01:00)
console.log(res1.toString());  // Sun Mar 28 2060 03:00:00 GMT+0200 (GMT+02:00)

// OK
const date2 = new Date(2060, 2, 27, 3);
const res2 = addDays(date2, 1);
console.log(date2.toString()); // Sat Mar 27 2060 03:00:00 GMT+0100 (GMT+01:00)
console.log(res2.toString());  // Sun Mar 28 2060 03:00:00 GMT+0200 (GMT+02:00)

// also OK as expected, we didn't cross over to DST
const date3 = new Date(2060, 2, 27, 1);
const res3 = addDays(date3, 1);
console.log(date3.toString()); // Sat Mar 27 2060 01:00:00 GMT+0100 (GMT+01:00)
console.log(res3.toString());  // Sun Mar 28 2060 01:00:00 GMT+0100 (GMT+01:00)

[fturmel]

Sorry everyone, I realized something interesting just now and it's not a bug after all.

The datetime you were landing on actually doesn't even technically exist because of the DST shift. The JS Date object resolves it to the next hour automatically.

process.env.TZ = 'CET';

console.log(new Date(2060, 2, 28, 1, 59, 59, 999).toString());
console.log(new Date(2060, 2, 28, 2).toString());
console.log(new Date(2060, 2, 28, 2, 30).toString()); // resolves to 3:30 am!
console.log(new Date(2060, 2, 28, 3).toString());

// Sun Mar 28 2060 01:59:59 GMT+0100 (GMT+01:00)
// Sun Mar 28 2060 03:00:00 GMT+0200 (GMT+02:00)
// Sun Mar 28 2060 03:30:00 GMT+0200 (GMT+02:00)
// Sun Mar 28 2060 03:00:00 GMT+0200 (GMT+02:00)

[punksta]

Thank you @fturmel
This is very interesting. That made me want to understand how DST shift works and how it's implemented in js 💯

[punksta]

In case you are curious how I ended up with these dates.

I was writing tests with https://github.com/dubzzz/fast-check for operations with Date objects and it generated these dates.