I would argue that at least for email ("External Targets"), we should always include the timezone when rendering times. When we allowed custom time-format, I've also added it there (Partly because there wasn't the time-zone warning we have now, partly because I kept my phabricator config to PST when traveling, and partly to make copying the time easier).

I think this is a legitimate bug where the viewer's timezone on the event becomes stuck as the last invitee we process, so events with multiple invitees in different timezones can generate mail with the last user's timezone.

I agree that we should always include timezone information in mail. We do for ModularTransactions now, but this mail doesn't use the same logic.

Additionally, we currently render mail timezones as "1:00 AM (UTC+7)", but there's actually a "T" conversion which includes a timezone abbreviation for a given locale that we can use to make this a little more readable.

• After D17646, event reminder mail should render with timezone information.
• Additionally, timezone information in this mail and other modern ModularTransactions mail will now render with timezone abbreviations where available: (PDT) instead of (UTC-7), for example.
• I wasn't able to identify or reproduce any actual bug here. Hopefully the additional information in D17646 will make it clear what's happening. The expectation is that the mail you receive should use the same locale as your account uses in Settings → Date and Time → Timezone, and in all my local attempts to reproduce this that was the case. Note that if an event is scheduled in UTC but instances cross daylight savings time in a particular locale the local time of the event may change. I don't think that's what happened here, just mentioning it. See also T11073.