Inconsistent diff() with .format() output

[jathanasiou]

It seems that that dayjs operation for .format() is not reversible via parsing its output with dayjs(dayjs().format()), not regards to the diff()` function at least.

To demonstrate, I experimented with the following snippet.

// Node.js v12.20.1
// "dayjs": "^1.10.4"
// OS Windows 10
const dayjs = require('dayjs');

const now = dayjs();

const futureDayjs = dayjs().add(49, 'day');
const futureString = futureDayjs.format();

console.log(`Now <diff> daysjs date: ${now.diff(futureDayjs, 'day')}`);
console.log(`Now <diff> string date: ${now.diff(futureString, 'day')}`);
console.log(`Now <diff> dayjs(string) date: ${now.diff(dayjs(futureString), 'day')}`);

And getting the output

Now <diff> daysjs date: -49
Now <diff> string date: -48
Now <diff> dayjs(string) date: -48

Which is not what I'd expect. Is there some timezone or other attribute that I'm missing that could be causing this or is this a bug?

[iamkun]

please pass true as the second argument of .diff

[jathanasiou]

please pass true as the second argument of .diff

console.log('with true')
console.log(`Now <diff> daysjs date: ${now.diff(futureDayjs, 'day', true)}`); // -49.00000002314815
console.log(`Now <diff> string date: ${now.diff(futureString, 'day', true)}`); // -48.99999065972222

So, while this does highlight the rounding issue, it still seems inconsistent that diff is providing different results for (arguably) the same date.

This can be very important for some edge cases as I discovered this during unit testing.

[iamkun]

this is because you just drop the millisecond in this line

const futureString = futureDayjs.format();

[jathanasiou]

Yeah that makes sense now that I think about it. I should be able to write tests that simply do not take MS into account then. Thanks!