# City-pair clock gaps, 2026–27

Companion notes for `wave3-clock-gaps-2026.csv`, published with the Kluro Research article [Recurring calls this autumn: whose clock moves, and on which date](/research/recurring-calls-clock-changes/).

## Purpose

For 13 city pairs, the CSV gives how many hours city B's clock is ahead of city A's, and the UTC window in which that gap holds, from 1 October 2026 to 30 April 2027. Use it to see when a standing call between two places shifts by an hour.

## Source and version

- IANA Time Zone Database (tzdb) release **2026e**, released 29 September 2026 (public domain): https://data.iana.org/time-zones/releases/tzdata2026e.tar.gz
- Computed 30 September 2026.

## Method

tzdata 2026e was compiled with `zic`, and UTC offsets were read with Python 3.11 `zoneinfo` every 15 minutes from 2026-10-01T00:00Z to 2027-04-30T00:00Z. A new row starts whenever the gap between the two zones changes.

Cross-check: rerunning with tzdata 2026d (PyPI `tzdata` 2026.4) and with macOS system time-zone data 2026c gave identical rows except the three Winnipeg pairs, whose permanent UTC−5 rule first appears in 2026e.

Device check (same day): the Mac used for this analysis (macOS 26.3, build 25D125) had system time-zone data 2026c. Apple's Foundation framework reported America/Winnipeg at UTC−6 on 2026-11-15, i.e. without Manitoba's 2026e rule.

## Fields

| Field | Meaning |
|---|---|
| city_a, city_b | Illustrative city for each zone (Seattle = America/Los_Angeles, Calgary = America/Edmonton) |
| iana_zone_a, iana_zone_b | tzdb zone names |
| hours_b_ahead_of_a | UTC offset of zone B minus UTC offset of zone A, in hours (a negative value would mean B is behind) |
| window_start_utc | First instant the gap applies (ISO 8601, UTC) |
| window_end_utc_exclusive | First instant the gap no longer applies (ISO 8601, UTC) |
| tz_database_version | tzdb release used |

## Units and null convention

- Gaps are in hours; timestamps are UTC in ISO 8601 with a `Z` suffix.
- An empty `window_end_utc_exclusive` means the gap continues past 2027-04-30T00:00Z, the end of the scan.
- The first row for each pair starts at the scan start (2026-10-01T00:00Z), not at the date that gap began.

## Limits

- 2027 rows assume current law (US 15 U.S.C. 260a; European and Australian rules as encoded in tzdb).
- tzdb temporarily models the British Columbia, Alberta, Northwest Territories and Manitoba changes as taking effect on 2026-11-01 at 02:00 local time (Manitoba's legal date is 2026-10-31). The UTC offsets are the same either way.
- A zone is not every community in a province: eastern BC mountain-time communities are outside America/Vancouver, and tzdb 2026e assumes parts of northwestern Ontario follow America/Winnipeg pending confirmation.
- Device and calendar-service copies of the time-zone data may lag tzdb; a device without the new rules will show different times.