Our issue is not with abbreviations (-07/MST), and changing the abbreviation to MST will not fix the issue. ICU checks whether a rule applies (tm_isdst), and chooses the DST name if one does.
A change that would be compatible with current CLDR data is: Rule Vanc 2026 only - Mar 9 0:00 1:00 D # Zone NAME STDOFF RULES FORMAT [UNTIL] Zone America/Vancouver -8:12:28 - LMT 1884 -8:00 Vanc P%sT 1987 -8:00 Canada P%sT (any time before 2026-11-01) -8:00 Vanc (P%sT, MST, -07, whatever) This keeps tm_isdst true for eternity; that's obviously not ideal, but we can come back and fix this when we know more, and when CLDR can handle such a change. On Fri, 6 Mar 2026 at 19:39, Paul Eggert <[email protected]> wrote: > On 2026-03-06 08:11, Robert Bastian via tz wrote: > > Because CLDR keys its display names by this flag, this change will cause > ICU and > > other downstream software (including major operating systems) to display > > BC’s UTC-7 as "Pacific Standard Time," while Los Angeles’ UTC-7 will > > display as "Pacific Daylight Time." > > Yes, and what's currently in the draft TZDB on GitHub (America/Vancouver > switching to abbreviation "-07") was intended to be merely a > placeholder. We weren't intending to publish it as 2026b. > > For the reason you mention and for other software-compatibility reasons, > let's change the "-07" placeholder to an alphabetic abbreviation. The > simplest fix appears to be to change to the abbreviation "MST", as is > already the case for America/Vancouver's geographic neighbors that > observe UTC-7 all year. This should fix the CLDR issue (albeit in a > different way than what you suggested), and should be good enough until > the BC government and/or common practice advises us all later on what > three-or-more-character abbreviation is most popular. > > Of course we'd prefer to make just one change now, and no changes later, > but it seems we do not have that luxury given the rushed way this was > done legally. And I would rather publish something before the actual > legal change occurs on Monday, rather than issue a post-facto change. >
