[tz] Yukon to move to year-round PDT

[Chris Walton]

>> I agree with Michael D.
>> 
>> There are four existing zones that are already on permanent UTC-7:
>> 	America/Dawson_Creek
>> 	America/Creston
>> 	America/Fort_Nelson
>> 	America/Phoenix
>> None of these zones makes use the DST bit.  The time zone is displayed year round as MST instead of PDT.  I don't think anybody complains about it.
>> 
>> I don't see any compelling reason to treat the two Yukon zones (America/Whitehorse & America/Dawson) any differently than the four existing UTC-7 zones.
>>  
>Historic timestamps?
Historic timestamps and their time zone abbreviations do not change.  Nobody is suggesting that they should be changed.
e.g. America/Dawson already supports all the historical time zone abbreviations that have been used in Yukon Territory since 1900:
      PDT, PST, YDDT, YDT, YPT, YST , & YWT.
The proposed change would force all dates that occur after March 14/2021 to have an MST time zone abbreviation.
All dates that occur before March 14/2021 would continue to be displayed with the correct historical time zone abbreviations.

The effect would similar to America/Fort_Nelson that switched from PST/PDT to permanent MST on March 8/2015.
$ TZ=America/Fort_Nelson date -d 'Mar 7 2015'
     Sat Mar  7 00:00:00 PST 2015
$ TZ=America/Fort_Nelson date -d 'Mar 9 2015'
     Mon Mar  9 00:00:00 MST 2015

-chris