# Primary source checks for the 2026 clock-change model

Access date for all sources: **2026-10-30**.

This file records external facts for a calendar model. It does not report research with people or customer calendars.

## Verified dates

| Location | Date when clocks go back | Local transition | Source basis |
| --- | --- | --- | --- |
| UK | 25 October 2026 | 02:00 BST becomes 01:00 GMT | GOV.UK |
| Mainland Portugal | 25 October 2026 | 02:00 becomes 01:00 | EU schedule and IPMA |
| Germany | 25 October 2026 | 03:00 CEST becomes 02:00 CET | Hamburg official portal and German ministry announcement |
| New York | 1 November 2026 | 02:00 becomes 01:00 | NIST national rule, applied to this city |
| Chicago | 1 November 2026 | 02:00 becomes 01:00 | NIST national rule, applied to this city |
| Denver | 1 November 2026 | 02:00 becomes 01:00 | NIST national rule, applied to this city |
| San Francisco | 1 November 2026 | 02:00 becomes 01:00 | NIST national rule, applied to this city |

The European transitions occur at 01:00 UTC. The US transitions occur at different UTC times because each zone changes at 02:00 local time.

The model must use `America/Los_Angeles` for San Francisco. The other selected identifiers are `Europe/London`, `Europe/Lisbon`, `Europe/Berlin`, `America/New_York`, `America/Chicago`, and `America/Denver`.

## UK government

- URL: https://www.gov.uk/when-do-the-clocks-change
- Access date: 2026-10-30.
- Direct page access succeeded.
- The table gives 25 October 2026 and 31 October 2027 as autumn clock-change dates.
- The rule is the last Sunday in October. Clocks move back one hour at 02:00 local time.
- The UK returns to GMT after this change.
- Caution: The page heading describes the next spring change. Use the row with the correct year.

## US National Institute of Standards and Technology

- URL: https://www.nist.gov/pml/time-and-frequency-division/popular-links/daylight-saving-time-dst
- Access date: 2026-10-30.
- Direct page access succeeded.
- The page explicitly gives 1 November 2026 at 02:00 local time as the end of DST.
- The national rule ends DST on the first Sunday in November.
- At that point, 02:00 becomes 01:00. The repeated hour has two different UTC offsets.
- New York, Chicago, Denver, and San Francisco use this DST rule. The page does not contain a separate table for these cities.
- NIST lists exceptions, including Hawaii and most of Arizona. Do not describe the result as a rule for every US location.
- The US Department of Transportation regulates time zones and DST. NIST distributes official time under those rules.

## EU schedule through 2026

- URL: https://www.boe.es/buscar/doc.php?id=DOUE-Z-2021-70039
- Access date: 2026-10-30.
- Direct page access succeeded.
- Spain's official state gazette republishes the European Commission schedule under Directive 2000/84/EC.
- The schedule explicitly ends the 2026 summer-time period on Sunday, 25 October, at 01:00 UTC.
- This supplies the date for Germany and mainland Portugal.
- Caution: Direct EUR-Lex access returned a JavaScript verification page. This check used the official BOE text instead.
- EUR-Lex URL attempted: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:52021XC0427(01)

## Mainland Portugal

- URL: https://www.ipma.pt/pt/educativa/hora/index.html
- Access date: 2026-10-30.
- Direct page access succeeded.
- IPMA states that mainland Portugal moves clocks back 60 minutes at 02:00 legal time, which is 01:00 UTC, in October.
- Combine this local transition with the EU schedule for the exact 2026 date.
- Caution: Keep the article's scope as mainland Portugal. The Azores use a different local time at the transition.
- Supporting archive: https://oal.ul.pt/hora-legal/mudanca-da-hora/
- Search returned the OAL table with 25 October 2026. Direct access timed out.
- OAL warns that the site no longer receives updates. Do not use it as the only current source.

## Germany

- URL: https://www.hamburg.de/freizeit/jahreszeiten/zeitumstellung-304830
- Access date: 2026-10-30.
- Direct page access succeeded.
- The official city portal explicitly gives 25 October 2026. It states that clocks move from 03:00 to 02:00.
- Federal supporting URL: https://www.bundeswirtschaftsministerium.de/Redaktion/DE/Pressemitteilungen/2026/10/20261022-sommerzeit-endet.html
- The ministry search result gives the same date and transition in an announcement dated 22 October 2026.
- Direct access to that announcement returned HTTP 429. Use the accessible Hamburg page or EU schedule for the article citation.

## Current EU framework

- URL: https://www.consilium.europa.eu/en/policies/seasonal-time-changes/
- Access date: 2026-10-30.
- Direct page access succeeded.
- The Council describes the current last-Sunday rules for March and October.
- Its page says that the proposal to end seasonal changes has no final agreement.
- Caution: The page is marked as archived. Its stated last content update is 20 August 2026.
- The explicit EU schedule supports the 2026 date more directly than this policy summary.

## Python model provenance

- URL: https://docs.python.org/3/library/zoneinfo.html
- Access date: 2026-10-30.
- Direct page access succeeded. A Context7 query also checked the CPython documentation and source.
- Source URL: https://github.com/python/cpython/blob/main/Doc/library/zoneinfo.rst
- `ZoneInfo` searches `TZPATH` for a zone file first. It uses the `tzdata` package if the search fails.
- If both sources lack the zone, `ZoneInfoNotFoundError` occurs.
- An installed `tzdata` package does not prove that a run used that package.
- `PYTHONTZPATH=""` makes a fresh process ignore system data and use the package.
- Record the Python version, selected data source, package version, IANA version, zone identifiers, and run date.
- If the model uses system data, identify that database separately. Do not label it with an unused package version.
- The `fold` field distinguishes repeated local times. Use UTC conversion or an explicit ambiguity policy when a model includes the repeated hour.

## IANA release provenance

- URL: https://www.iana.org/time-zones
- Release URL: https://www.iana.org/time-zones/releases/2026f
- Data download URL: https://data.iana.org/time-zones/releases/tzdata2026f.tar.gz
- Access date: 2026-10-30.
- Direct page access succeeded.
- IANA lists **2026f**, released **27 October 2026**, as its latest release on this access date.
- IANA updates the database when governments change time-zone boundaries, UTC offsets, or DST rules.
- Caution: The latest upstream release does not establish the version installed on this computer.
- Use a fixed release and record its provenance if the model must be reproducible.

## Year and claim controls

- **1 November 2026 is a Sunday.** It is the US change date for the four selected cities.
- **1 November 2027 is a Monday.** It is not the US change date under the current rule.
- Under that rule, the US change date in 2027 is **7 November**.
- GOV.UK gives **31 October 2027** for the UK change.
- Python's calendar module confirmed both November weekdays and the first Sunday of each year.
- Do not reuse 2026 dates in a 2027 article or chart.
- As of 30 October 2026, the selected European cities have changed. The selected US cities have not changed yet.
- A model can calculate shared office hours under stated assumptions. It cannot establish observed free slots, missed meetings, revenue loss, or staff productivity.
- Label calculated results as model results. Keep the workday, lunch, meeting duration, and recurrence assumptions explicit.
