Section 7: Every Monday at Nine, Wherever Nine Is - Deep Dive: Recurrence, Time Zones & Exceptions

Executive Summary: A recurring event is a rule, not a row. We cover RFC 5545's RRULE, RECURRENCE-ID and EXDATE, store Priya's series as New York wall-clock time plus an IANA zone, and walk the DST fortnight in which Tomás sees 14:00, 13:00 and 14:00. Then come the edge cases (missing and doubled times, floating dates, "last Friday", the 31st), the three edit scopes including the atomic :split, deterministic expand-on-read, and two pushbacks: "just materialize it" and "a country changed its DST rules".

The interviewer points at Platform Sync on the board: "Show me how 'every Monday at 9' actually works. Not the boxes. The event."

The Rule, in Its Own Words

Priya's series is three iCalendar lines:

DTSTART;TZID=America/New_York:20261019T090000
DTEND;TZID=America/New_York:20261019T094500
RRULE:FREQ=WEEKLY;BYDAY=MO

FREQ sets the cadence and BYDAY picks the day. COUNT or UNTIL would end the series; Platform Sync has neither, so its series_end_utc is ∞ (Section 5's index). Two more properties edit a running series. EXDATE lists original starts that are cancelled. A RECURRENCE-ID marks an override, a separate component that replaces one instance and is keyed by that instance's original start.

You: "The part people skip is TZID. The 09:00 is a New York wall-clock time, not an instant. Every instance is computed in New York and only then converted to UTC."