Time zone rules at a date
The offset, abbreviation, and daylight saving state of an time zone at a moment, and when it next changes.
Clocks there read MDT, which is -06:00 from UTC.
- Abbreviation
- Daylight saving
- Next change (UTC)
- Offset after the change
- Zone
Provenance
- Computed by
- time.scale.zone-info 1.0.0, core 0.1.0
- Model
- Embedded IANA tzdb (TZif with POSIX rules past the last listed transition)
- Accuracy
- Exact for the tzdb release echoed in meta.assets; checked against Python zoneinfo at 238,800 instants
- Reference data
- version 2026d of a reference dataset
- Notes
- None
- Cites
- Internet Assigned Numbers Authority, Time Zone Database (tzdb)
Something look off?
How we got thisFormula, worked example, sources, and proof
Model: Embedded IANA tzdb (TZif with POSIX rules past the last listed transition)
Accuracy: Exact for the tzdb release echoed in meta.assets; checked against Python zoneinfo at 238,800 instants
When to use this: Use this to answer what the clocks read somewhere at a given moment: the UTC offset, the abbreviation, whether daylight saving is in force, and when it next changes. It is the lookup to reach for before scheduling anything across zones, or before reading a local timestamp in someone else's records.
Limitations: A zone's rules are legislation, not arithmetic, and they change -- sometimes weeks before they take effect. The answer is as current as the embedded tzdb release, which is reported with every result so it can be checked. Past the last transition listed for a zone, the answer comes from that zone's POSIX rule string, which is what the tzdb itself projects forward. This takes an instant and returns the local reading; it does not turn a local wall-clock time into an instant, which is ambiguous twice a year and impossible once.
Worked example: Denver in September 2026. Source: IANA tzdb 2026d: MDT (-06:00) until 2026-11-01 08:00 UTC. It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.
You enter
- At (UTC)
- 2026-09-18T00:00Z
- Time zone
- America/Denver
You get
- UTC offset
- -06:00
- Abbreviation
- MDT
- Daylight saving
- yes
- Next change (UTC)
- 2026-11-01T08:00:00Z
- Offset after the change
- -07:00
- Zone
- America/Denver
Review: Not yet independently reviewed by a geodesist.
Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.
Status: version 1.0.0, core 0.1.0. See this tool in the verification report.
Checked against: 26 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.
Sources
- Time Zone Database (tzdb), Internet Assigned Numbers Authority, Release 2026d, compiled with zic -b slim. Zone and Link entries for every region; TZif per RFC 8536.
Terms
- IANA: Internet Assigned Numbers Authority
- The body that publishes the time zone database of names like America/Denver, which this site embeds. Source: IANA Time Zone Database
- UTC: Coordinated Universal Time
- The world's civil time standard, kept within 0.9 s of Earth's rotation by leap seconds. Aviation calls it Zulu time, written with a Z. Source: IERS Bulletin C (leap seconds)
IANA — Internet Assigned Numbers Authority
The body that publishes the time zone database of names like America/Denver, which this site embeds.
Source: IANA Time Zone Database
UTC — Coordinated Universal Time
The world's civil time standard, kept within 0.9 s of Earth's rotation by leap seconds. Aviation calls it Zulu time, written with a Z.
Source: IERS Bulletin C (leap seconds)
Learn the concept: Zulu time (UTC) explained