On 6/7/19 11:57 AM, Paul Gilmartin via tz wrote:
States may likewise choose to move one timezone eastward. Either action requires only approval by the Secretary of Transportation, not an act of Congress. The Secretary's authority to do that is not unlimited. Changes to time zone boundaries by the Secretary must have "regard for the convenience of commerce and the existing junction points and division points of common carriers engaged in interstate or foreign commerce" according to the US Code. The DOT's website for this <https://www.transportation.gov/regulations/procedure-moving-area-one-time-zo...> suggests that they probably wouldn't approve the request of a whole state merely on the grounds that the state wants to change. My guess is that this is because such a change could be challenged in court on the grounds that the confusion would hurt commerce and thus contradict the law.
What is the effect on tm_isdst? Do they care whether their documents are timestamped PDT or MST?
As far as I know these questions have not been addressed, and quite possibly they won't be addressed even if the law changes so we'll just have to do our best. If (say) California changes to permanent -07 then the longstanding tzdb tradition would be to mark it as standard time (i.e., tm_isdst = 0); presumably it would be abbreviated "MST" for compatibility with Internet RFC 5322 and the like. No matter what we put into tzdb at that point, we'd undoubtedly break something and get complaints. There are similar issues in the upcoming changes to European time zones.