fix(calendar): correct day-of-week for full-day recurring events across all timezones

[KristjanESPERANTO]

Fixes full-day recurring events showing on wrong day in timezones west of UTC (reported in #4003).

Root cause: moment.tz(date, eventTimezone).startOf("day") interprets UTC midnight as local time:

• 2025-11-03T00:00:00.000Z in America/Chicago (UTC-6)
• → Converts to 2025-11-02 18:00:00 (6 hours back)
• → .startOf("day") → 2025-11-02 00:00:00 ❌ Wrong day!

Impact: The bug affects:

• All timezones west of UTC (UTC-1 through UTC-12): Americas, Pacific
• Timezones east of UTC (UTC+1 through UTC+12): Europe, Asia, Africa - work correctly
• UTC itself - works correctly

The issue was introduced with commit c2ec6fc (#3976), which fixed the time but broke the date. This PR fixes both.

	Result	Day	Time	Notes
Before c2ec6fc	2025-11-03 05:00:00 Monday	✅	❌	Wrong time, but correct day
Current (c2ec6fc)	2025-11-02 00:00:00 Sunday	❌ (west of UTC) ✅ (east of UTC)	✅	Wrong day - visible bug!
This fix	2025-11-03 00:00:00 Monday	✅	✅	Correct in all timezones

Note: While the old logic had incorrect timing, it produced the correct calendar day due to how it handled UTC offsets. The current logic fixed the timing issue but introduced the more visible calendar day bug.

Solution

Extract UTC date components and interpret as local calendar dates:

const utcYear = date.getUTCFullYear();
const utcMonth = date.getUTCMonth();
const utcDate = date.getUTCDate();
return moment.tz([utcYear, utcMonth, utcDate], eventTimezone);

Testing

To prevent this from happening again in future refactorings, I wrote a test for it.

npm test -- tests/unit/modules/default/calendar/calendar_fetcher_utils_spec.js

[sdetweil]

Is the a matching edge case with one hour close to
DST which should move forward , but doesn’t

[KristjanESPERANTO]

I initially thought this was DST-related too, but after testing I just updated the PR text - it's not about DST.

The bug affects all full-day events in western timezones (UTC-1 to UTC-12), regardless of DST:

// Western timezones: Always wrong
moment.tz(new Date("2025-11-03T00:00:00Z"), "America/Chicago")
// → 2025-11-02 18:00 → .startOf("day") → 2025-11-02 ❌

// Eastern timezones: Always correct
moment.tz(new Date("2025-11-03T00:00:00Z"), "Asia/Tokyo")  
// → 2025-11-03 09:00 → .startOf("day") → 2025-11-03 ✅

No matching edge case exists - the bug is asymmetric. UTC midnight converts to the previous day in western timezones, but same day in eastern timezones. That's why eastern timezones work correctly by design.

[KristjanESPERANTO]

I just fixed a comment that still referred to DST. No changes in the logic.

[KristjanESPERANTO]

I'm too tired to continue researching right now, but after further investigation, it seems to me that the old logic before c2ec6fc is not as wrong as I described it above 🤯. But that doesn't change the fact that we need a fix (this PR) now.

I hope that we switch to Temporal at some point, then all the time zone stuff will be a little less confusing.

[sdetweil]

No rush. Not shipping til April 1

[sdetweil]

User reports PR fixed issue

[KristjanESPERANTO]

Great 🙂 Then we can merge it, right?

[KristjanESPERANTO]

@sdetweil Is the problem serious enough that we should do a fix release? 🤔

[sdetweil]

no, we can teach them how to get the PR

[KristjanESPERANTO]

Okay, I think that's a good call 👍