Wrong diff for DST in UTC mode

[jkondela]

Describe the bug
When using utc and one date is in DST while another is in non-DST, diff returns wrong value.
Diff for hours is correct, only days diff is affected.

For comparison, moment works as expected.

I noticed that diff is using timezone delta, but when dayjs is in UTC mode, the delta is 0 which is probably causing this bug.
https://github.com/iamkun/dayjs/blob/dev/src/index.js#L314

Maybe the same problem as here #1021

const dayjsD1 = dayjs.utc("2020-10-31"); // non DST date
const dayjsD2 = dayjs.utc("2020-10-31").add(-100, "days"); // DST
console.log(dayjsD1.diff(dayjsD2, "days"))
// prints 99


const momentD1 = moment.utc("2020-10-31");
const momentD2 = moment.utc("2020-10-31").add(-100, "days");
console.log(momentD1.diff(momentD2, "days"))
// prints 100

Expected behavior
Subtract 100 days from the original date should return diff 100 days.

Information

• Day.js Version v1.9.1
• OS: Windows 10
• Browser Chrome 85, Firefox 81
• Time zone: GMT+2 DST Bratislava (Central European Summer Time)

Thanks for the excellent date library.

[LiPinghai]

+1

[pavel-klavik]

+1, noticed this problem due to recent DST change in Central Europe. The number of hours is computed correctly by diff, so I can use it as a workaround.

const day1 = dayjs.utc("20201023");
const day2 = dayjs.utc("20201026");
day1.diff(day2, "d"); // incorrect 2
day1.hour(day2, "h"); // correct 72

[iamkun]

fixed, thanks.

[iamkun]

🎉 This issue has been resolved in version 1.9.5 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[robozb]

I experienced subtract error when I use timezones, an example:

import dayjs from 'dayjs'; 
import utc from 'dayjs/plugin/utc.js'
import timezone from 'dayjs/plugin/timezone.js'

dayjs.extend(utc); 
dayjs.extend(timezone);

const dt1=dayjs().tz("Atlantic/Reykjavik")
console.log(dt1.format("YYYY-MM-DD HH:mm"))

const dt2=dt1.subtract(1,"day")
console.log(dt2.format("YYYY-MM-DD HH:mm"))

Result:

2023-08-24 00:38
2023-08-22 22:38