Unexpected hour/UTC offset across daylight savings time

Adjusting a Dayjs instance for a new timezone across daylight savings time results in unexpected hour and UTC offset.

let date = dayjs('2021-03-17T09:00:00Z');
console.log(date.format('HH:mm DD MMM YY Z'));
date = date.tz('Etc/GMT-6', false);
console.log(date.format('HH:mm DD MMM YY Z'));
date = date.tz('Etc/GMT-6', true);
console.log(date.format('HH:mm DD MMM YY Z'));

**Actual behavior**
05:00 17 Mar 21 -04:00
15:00 17 Mar 21 +05:00
15:00 17 Mar 21 +05:00

**Expected behavior**
I would expect the last date to retain the hour and UTC offset from the original date since keepLocalTime flag was set to true.
05:00 17 Mar 21 -04:00
15:00 17 Mar 21 +05:00
05:00 17 Mar 21 +06:00

**Information**
- Day.js Version 1.10.4
- OS: Window
- Browser Chrome Version 88.0.4324.190 (Official Build) (64-bit)
- Time zone: Eastern Standard Time, GMT-5

No source file or test is named. Start by running the supplied Day.js timezone reproduction with the stated environment and inspect the .tz handling for keepLocalTime across daylight-saving offsets; done means the final output retains 05:00 and reports +06:00 as expected, with coverage for this case.

Written by the indexing model from the issue text.