# IANA Time Zone Transition List

> Lists every UTC-offset transition for one IANA zone and date range using pinned tzdb 2026d, including the local boundaries, offsets, abbreviations and whether the clock creates a gap or overlap.

- Calculator id: `time-zone-transitions` · Category: Time & Geography (`time`) · Tool name: `list_time_zone_transitions`
- Canonical page: https://tttkmbb.com/time/time-zone-transitions · This document: https://tttkmbb.com/time/time-zone-transitions.md · JSON definition: https://tttkmbb.com/time/time-zone-transitions.json

## Purpose

Lists every UTC-offset transition for one IANA zone and date range using pinned tzdb 2026d, including the local boundaries, offsets, abbreviations and whether the clock creates a gap or overlap.

**Use when:** You need the next or historical daylight-saving/offset changes for scheduling, payroll, travel, audits or a regression test, with a specific reproducible tzdb version.

**Do not use when:** You only need to convert one ordinary date-time, need legal advice about a jurisdiction's time law, or need changes announced after this pinned release.

## Example user requests

- List every 2026 UTC-offset transition in America/New_York with local boundaries.
- When does Australia/Adelaide change clocks between 2026-01-01 and 2027-01-01?
- 列出 Europe/London 在 2026 年的夏令时转换点、前后偏移和 IANA 数据版本。


## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `time_zone` | string |  | required | IANA zone identifier present in tzdb 2026d. |
| `start_date` | date |  | required | First local calendar date to inspect (1900-01-01 through 2100-12-31). |
| `end_date` | date |  | required | Last local calendar date to inspect; ranges may not exceed 3,660 days. |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `time_zone` | string |  | Zone identifier supplied by the caller. |
| `resolved_zone_id` | string |  | Zone record used by the pinned data build. |
| `start_date` | date |  | First local date inspected. |
| `end_date` | date |  | Last local date inspected. |
| `transition_count` | integer |  | Number of offset changes in the selected local-date range. |
| `transitions` | list |  | UTC instant, gap/overlap kind, local boundaries, abbreviations and offsets before/after. |
| `tzdb_version` | string |  | Pinned IANA release used for every offset and transition. |
| `dataset_version` | string |  | Immutable Calcgrid evidence-package version. |
| `source_url` | string |  | Version-specific official IANA release URL. |
| `source_sha256` | string |  | SHA-256 of the downloaded official IANA tzdata release artifact. |
| `coverage_note` | string |  | Coverage and rule-change limitations for this fixed version. |

## Formula

`select zone transition instants whose local boundary before or after lies inside [start_date, end_date]`

Reads the immutable transition array embedded from IANA tzdb 2026d; no live network call or Worker runtime offset lookup is involved.

## Data Sources

- IANA Time Zone Database 2026d — https://data.iana.org/time-zones/releases/tzdata2026d.tar.gz (standard, retrieved 2026-09-26)
- IANA tz database LICENSE — https://data.iana.org/time-zones/tzdb/LICENSE (license, retrieved 2026-09-26)
- Moment Timezone 0.6.4 data build — https://www.npmjs.com/package/moment-timezone/v/0.6.4 (implementation, retrieved 2026-09-26)

Data freshness: `static`. Immutable IANA tzdb 2026d evidence package. Future government changes require a new version.

## API

- `GET https://tttkmbb.com/api/v1/calculate/time-zone-transitions?time_zone=…&start_date=…&end_date=…`
- `POST https://tttkmbb.com/api/v1/calculate/time-zone-transitions` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/time-zone-transitions · OpenAPI operationId `list_time_zone_transitions` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool: `list_time_zone_transitions` (dedicated) or `run_calculator` with `{"calculator_id": "time-zone-transitions", "inputs": {…}}`

## Example

- New York transitions in 2026: inputs `{"time_zone":"America/New_York","start_date":"2026-01-01","end_date":"2026-12-31"}` → `{"transition_count":2,"transitions":[{"utc_datetime":"2026-03-08T07:00:00Z","kind":"gap","utc_offset_before":"-05:00","utc_offset_after":"-04:00"},{"utc_datetime":"2026-11-01T06:00:00Z","kind":"overlap","utc_offset_before":"-04:00","utc_offset_after":"-05:00"}],"tzdb_version":"2026d"}`

```
GET https://tttkmbb.com/api/v1/calculate/time-zone-transitions?time_zone=America%2FNew_York&start_date=2026-01-01&end_date=2026-12-31
```

## Limitations

You only need to convert one ordinary date-time, need legal advice about a jurisdiction's time law, or need changes announced after this pinned release. Reads the immutable transition array embedded from IANA tzdb 2026d; no live network call or Worker runtime offset lookup is involved. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Does an empty list mean the zone is invalid?**

No. A valid zone can have no offset changes in the requested range; transition_count will be zero.

**Can the future transitions change?**

Yes. Governments can change rules after tzdb 2026d; use the returned version and source fields and check the data index for updates.

## Related

- [IANA Local Time Resolver](https://tttkmbb.com/time/local-time-resolver.md) — Validate a wall time inside a transition gap or overlap.
- [Time Zone Converter](https://tttkmbb.com/time/time-zone-converter.md) — Convert a wall time between zones using the same pinned release.
- [World Clock Calculator](https://tttkmbb.com/time/world-clock.md) — Show one instant in several zones.