Pyas calendarsThree Canadian time zones Outlook uses that older Windows-to-IANA mappings lack
Updated
TL;DR
Windows, and so Outlook, has three time zone names for Canadian provinces that moved to year-round time in 2026: British Columbia Standard Time, Alberta Standard Time and Manitoba Standard Time. CLDR's Windows-to-IANA table only maps them from release 49 (October 21, 2026), so code built on an earlier table finds no match and usually falls back to UTC. Map them to America/Vancouver, America/Edmonton and America/Winnipeg, and keep your time zone data current.
What we saw
Microsoft Graph describes time zones with Windows names, such as "Pacific Standard Time", unless you ask for something else. Most code converts them to IANA names (America/Los_Angeles) with the Windows-to-IANA table in the Unicode CLDR project.
Found while mapping Outlook's zone list, 2026-10-26
Outlook offers 143 Windows time zone names. Three of them, which Windows added for the provinces' year-round time, were not in CLDR's table at the time (release 48): British Columbia Standard Time, Alberta Standard Time and Manitoba Standard Time. A mailbox set to one of them had its events read in UTC.
The mapping
| Windows name | IANA zone |
|---|---|
| British Columbia Standard Time | America/Vancouver |
| Alberta Standard Time | America/Edmonton |
| Manitoba Standard Time | America/Winnipeg |
CLDR release 49, published on October 21, 2026, adds all three. Libraries and runtimes ship CLDR data on their own schedule, so expect older tables in the wild for a while.
The IANA zones behind them changed this year too. The tz database records British Columbia moving to permanent UTC-7 in March 2026, Alberta to permanent UTC-6 in June, and Manitoba to permanent UTC-5 on October 31, 2026 (tz 2026e). A runtime with older tz data still changes the clocks in Winnipeg in November, an hour off from what Outlook shows. Check which tz version your runtime has, and update it.
What to do
- Add the three names to whatever Windows-to-IANA table you use, unless it is built on CLDR 49 or later.
- Keep tz data current. Zone rules change by law, sometimes with weeks of notice.
- Never fall back to UTC silently. When a Windows name is unknown, use the mailbox's own time zone from
mailboxSettings, and log the name so you can add it. - Test zones beyond your own market. Half-hour and 45-minute offsets (India, Nepal, Newfoundland, the Chatham Islands) and southern-hemisphere clock changes are where conversions break.
How Pyas handles it
Pyas maps all three names, along with CLDR's list, so every Outlook event comes back with an IANA time zone and a local time. A zone Outlook names in a way no table knows, such as "Customized Time Zone", falls back to the mailbox's own zone instead of UTC.