`dayjs.tz` is not idempotent with dates in DST when it's not currently DST

Describe the bug Given:

• it is currently Standard Time (i.e. not Daylight Saving Time)
• the date being manipulated is in Daylight Saving Time

When Dayjs#tz is called more than once on a Dayjs object, the time moves back 1 hour each time Dayjs#tz is called.

> dayjs('2020-08-08T00:00:00.000Z').toString()
//=> 'Sat, 08 Aug 2020 00:00:00 GMT'

> dayjs('2020-08-08T00:00:00.000Z').tz('America/Chicago').toString()
//=> 'Fri, 07 Aug 2020 23:00:00 GMT'

> dayjs('2020-08-08T00:00:00.000Z').tz('America/Chicago').tz('America/Chicago').toString()
//=> 'Fri, 07 Aug 2020 22:00:00 GMT'

> dayjs('2020-08-08T00:00:00.000Z').tz('America/Chicago').tz('America/Chicago').tz('America/Chicago').toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'

The result is the same regardless of what timezones are used, or whether a timezone arg is passed in at all:

> dayjs('2020-08-08T00:00:00.001Z').tz('UTC').tz().tz('UTC').toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'

> dayjs('2020-08-08T00:00:00.001Z').tz('America/Chicago').tz('UTC').tz().toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'

> dayjs('2020-08-08T00:00:00.001Z').tz().tz().tz().toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'

It’s February and I’m in Chicago, so I’ve only been able to confirm this issue occurs during CST with a date in CDT. I suspect that the opposite will be true in a few weeks (that calling Dayjs#tz on a CST time during CDT will result in a one-hour shift the other way).

And I saw several open issue tickets that are likely related or the result of the same underlying problem:

• #1804
• #1803
• #1801
• #1795
• #1791
• #1690
• #1664
• #1657
• #1635
• #1622
• #1260
• #1001

Expected behavior Applying a timezone should not change the underlying time.

Information

• Day.js v1.10.7
• OS: iOS 12.1
• Time zone: GMT-06:00 (Central Standard Time)

[sulkaharo]

@iamkun can you confirm is the library is still being maintained? Based on the commit history you’ve touched dayjs logic last time 7 months ago and there doesn’t seem to be another developer on board maintaining dayjs, resulting in a long backlog of pull requests and issues. If you’ve burned out or don’t have time for this library, let’s find a solution to getting the development back on track.

[justingolden21]

+1 to what @sulkaharo said

@iamkun if all of the PRs and issues and noise around this project is too much, we should find a way to keep the development of this project going, since there are a lot of capable and willing people and it’s a widely used (and awesome) library.