Local time to UTC (Zulu)

Model: UTC = local − offset; named zones from the embedded IANA tzdb (TZif with POSIX rules)

Show your work

1. Offset from UTC

   the zone's offset at that moment, daylight saving included

   -05:00 = -5 hours

2. The same moment in UTC

   UTC = local time − offset

   2026-07-01T14:05:00-05:00 = 2026-07-01T19:05:00Z

3. Zulu

   the same time written the way a flight plan wants it

   2026-07-01T19:05:00Z = 1905Z

The same steps an agent gets from the MCP server with explain: true.

Accuracy: Exact for the tzdb release echoed in meta.assets

When to use this: Use this when a time has to be right across zones: filing or reading a flight plan in Zulu, converting a log entry, scheduling an observation, or checking what a local time means in UTC on a date when daylight saving shifts. It takes either a named IANA zone or a plain offset, and shows the date change when one happens.

Limitations: The zone rules come from the embedded IANA database, whose release is reported with the result; a change published after that release is not in it. A local time in a spring-forward gap does not exist and one in a fall-back overlap happens twice, and this reports that rather than choosing for you. It does not read your computer's clock or its time zone.

Worked example: 14:05 in Chicago on a summer date. Source: add-practitioner-essentials Zulu scenario: 1905Z. It is golden test vector v011, and every build checks the tool still gives its answer within its tolerance.

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 1.1.0, core 0.1.0. See this tool in the verification report.

Changes

• 2026-09-23, fixed: Converting UTC to local time, the sentence repeated the Zulu time you entered ("That is 2300Z.") instead of the answer. It now gives the local time and zone ("That is 15:00 AKDT."), and when the date changes it gives the local date. Three outputs are new: answer (the converted time in the direction asked), to_local (1 for UTC to local, 0 for local to UTC), and local_date_change. Local to UTC reads as before. The tool is now version 1.1.0. Changelog
• 2026-09-19, changed: The four aviation nights and local time to Zulu are now stable. The nights match 491 regulatory windows built from US Naval Observatory times at 100 places; Zulu matches Python zoneinfo on 2,388 instants and the Python documentation's Los Angeles examples. Results are unchanged. Changelog
• 2026-09-19, fixed: Short and legacy tzdb zone names such as CET, EET, and EST5EDT are now accepted wherever a time zone is asked for; before, they were refused as malformed UTC offsets. Changelog
• 2026-09-18, result change: Named IANA time zones like America/Denver are supported, so inputs that returned UNSUPPORTED now resolve to local times. Changelog

Checked against: 24 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• RFC 3339, Date and Time on the Internet: Timestamps, Internet Engineering Task Force, Proposed standard. Section 5.6 (timestamp format with a numeric UTC offset or Z).
• 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

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)