The mismatch mechanics
The US and Europe switch DST about 2–3 weeks apart each spring and fall; the southern hemisphere runs opposite seasons entirely. During the mismatch windows, the offset gap between New York and London is 4 hours instead of 5 — and every recurring meeting scheduled in local time silently shifts for exactly one side.
The UTC defense
Schedule recurring global meetings in UTC (or any fixed anchor) and display local times dynamically — the meeting holds its absolute position while local displays adapt. The cost: twice a year, someone's local hour changes by one and the fairness needs a re-check. That is the entire price; the alternative is meetings that teleport for weeks.
The calendar checklist
Twice a year (mid-March, late October): re-verify recurring global meetings against the new offsets, re-run the fairness math, and confirm the DST dates of every involved continent. Fifteen minutes per season immunizes the calendar against the most reliable scheduling bug in global work.