[timezone] wrong toISOString() value with zone conversion and DST

[dzonatan]

Describe the bug

// 'Europe/Vilnius' timezone => GMT+2, DST+1

// zone initialization - ok
dayjs.tz('2022-01-01T00:00:00.000Z', 'Europe/Vilnius').format()
// OUTPUT: '2022-01-01T00:00:00+02:00' ✅
dayjs.tz('2022-01-01T00:00:00.000Z', 'Europe/Vilnius').toISOString()
// OUTPUT: '2021-12-31T22:00:00.000Z' ✅

// zone conversion - fails
dayjs.utc('2021-12-31T22:00:00.000Z').tz('Europe/Vilnius').format()
// OUTPUT: '2022-01-01T00:00:00+02:00' ✅
dayjs.utc('2021-12-31T22:00:00.000Z').tz('Europe/Vilnius').toISOString()
// OUTPUT: '2021-12-31T23:00:00.000Z' ❌
// EXPECTED: '2021-12-31T22:00:00.000Z'

FIY: moment.js handles this properly.

Information

• Day.js v1.10.7
• OS: macOS Big Sur

[mr-short]

I am seeing similar 1 hour off issues with timezones (compared to momentjs). Central time is -5/-6 but dayjs is giving 7 hour offset?

dayjs('2021-01-01').tz('America/Chicago').toISOString()
'2021-01-01T07:00:00.000Z'

moment('2021-01-01').tz('America/Chicago').toISOString()
'2021-01-01T06:00:00.000Z'