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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.

When to use

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。

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.

Inputs

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

Outputs

Output	Type	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.

Example

New York transitions in 2026: {"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

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/time-zone-transitions (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/time-zone-transitions · Markdown: https://tttkmbb.com/time/time-zone-transitions.md · JSON definition: https://tttkmbb.com/time/time-zone-transitions.json
MCP: server https://tttkmbb.com/mcp, tool list_time_zone_transitions
OpenAPI operationId: list_time_zone_transitions
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

IANA Time Zone Database 2026d (standard)
IANA tz database LICENSE (license)
Moment Timezone 0.6.4 data build (implementation)

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 calculators

IANA Local Time Resolver — Validate a wall time inside a transition gap or overlap.
Time Zone Converter — Convert a wall time between zones using the same pinned release.
World Clock Calculator — Show one instant in several zones.