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ROOT CAUSE IDENTIFIED: Daylight Saving Time Bug in Zoom API
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THE SMOKING GUN:
User reported: “Issues noticed in the last two weeks”
Date of report: January 28, 2026
Timeline:
- Meetings created: April-August 2025 (during MDT)
- Worked fine: April → November 2025
- DST ended: November 2, 2025 (clocks fell back)
- Problems started: ~January 2026 (2+ months after DST change)
THE PROBLEM:
Zoom’s API is returning INCORRECT UTC times for recurring meetings that:
- Were created during Daylight Saving Time (MDT, UTC-6)
- Are now generating occurrences during Standard Time (MST, UTC-7)
MATHEMATICAL PROOF:
Meeting ID 85345560457 configured for 5pm Denver:
CORRECT BEHAVIOR:
During MDT (summer): 5pm MDT = 5pm + 6 hours = 23:00 UTC (11pm)
During MST (winter): 5pm MST = 5pm + 7 hours = 00:00 UTC next day (midnight)
WHAT ZOOM IS DOING (WRONG):
Occurrences in Jan 2026: Returns 2026-01-30T06:00:00Z
This equals: 6am UTC = 11pm MST (previous day)
WHAT ZOOM SHOULD DO:
Occurrences in Jan 2026: Should return 2026-01-30T00:00:00Z
This equals: midnight UTC = 5pm MST (previous day) ✓
THE OFFSET PATTERN:
Meeting 85345560457: 06:00 UTC (should be 00:00)
- 6 hours too late
- This is exactly the MDT offset (UTC-6)
- Zoom is applying summer offset to winter occurrences!
Meeting 84538274892: 09:00 UTC
- 9 hours too late
- This suggests either:
- Meeting was created in Pacific Time (UTC-8) + DST issue
- Or double offset error
Meeting 89035466055: 00:00 UTC ✓ CORRECT
- Likely created during MST (winter)
- Or created recently after Zoom fixed their bug
- Or manually corrected in Zoom interface
SUPPORTING EVIDENCE:
-
User reports meeting “occurrence already passed”
- Trying to join at 5pm Thursday
- System thinks meeting was at 11pm Thursday (already happened)
- This is because Zoom returned 06:00 UTC = 11pm Wednesday
-
Yahoo email fix was unnecessary
- The issue wasn’t email format
- The issue was meeting times being wrong in the data
-
Timing of problem discovery
- Not noticed until January (2 months after DST change)
- Suggests gradual realization as affected meetings approached
-
Some meetings work, others don’t
- Pattern matches MDT vs MST creation time
WHY YOUR CODE IS CORRECT:
Your code is doing EXACTLY what it should:
Input: 2026-01-30T06:00:00Z (what Zoom sends)
Process: 6am UTC - 7 hours MST offset = 11pm previous day
Output: 2026-01-29 23:00:00 MST
This is mathematically perfect. The bug is in Zoom’s data.